經濟學

香港嘅維港嗰頭有好多商場，日日都有大量嘅貨同服務被生產、分配同消費。

東京築地市場有水產批發，都係經濟活動－好多人買海鮮，目的係滿足自己「想食海鮮」嘅慾望。

紐約證券交易所每日都做好多交易－有好多人買股票，用嚟滿足自己「想要第時多啲錢使」嘅慾望。

經濟學ging1 zai3 hok6（英文：economics ^[1]^[2]；希臘文：οἰκονομία，oikonomia），廣東話日常有陣時會嗌 ji6 kon1*6，係社會科學嘅一個領域，專研究貨品同服務嘅生產、消費同埋分配。經濟學嘅研究範疇包括個體人類嘅行為同互動，以至成個經濟體嘅運作，主要可以按照研究現象嘅規模分做兩大子領域：

微觀（micro）經濟學著重分析經濟體嘅基本組成元件同呢啲元件之間嘅互動－「基本元素」包括咗個人、家庭、公司、買家同賣家呀噉；而
宏觀（macro）經濟學會成個經濟體噉分析，研究一啲描述個經濟體嘅總體屬性嘅變數，好似係本地生產總值、失業率同通脹呀噉。

呢啲微觀嘅物體同埋經濟體嘅宏觀屬性仲會互相影響，形成各種各樣嘅經濟現象^[3]^[4]。

經濟學唔淨只會客觀噉研究經濟現象，仲會思考一個經濟體點先係「理想」：經濟學有得細分做集中思考「經濟客觀睇係點」嘅實證（positive）經濟學同埋集中思考「經濟應該係點」嘅規範（normative）經濟學；前者會嘗試理解經濟體跟從乜嘢法則運作，似一門科學，而後者就往往會運用前者嘅研究得出嘅結果，思考要點樣操作一個經濟體，先可以令個經濟體達到理想嘅狀態－「理想」呢家嘢係有少少主觀嘅。規範經濟學喺經濟政策嘅制定上扮演咗舉足輕重嘅角色^[5]^[6]。

經濟學研究既有理論性貢獻，又有實用價值^[7]^[8]。經濟學同多數嘅社會科學比起上嚟精確好多，好多時會運用數學方程式描述、模擬、同預測所研究嘅現象。所以學術界喺 1968 年創立諾貝爾經濟學獎，承認經濟學係一門嚴謹嘅科學同表揚呢個領域對人類嘅貢獻^[9]。

定位

歷史上嘅定義

現代嘅學界對「經濟學」有多個定義，當中有一啲反映學界對呢個領域嘅新認識，有啲反映咗唔同學派嘅見解^[10]^[11]。經濟學之父蘇格蘭哲學家亞當史密夫（Adam Smith）將「政治經濟學」（political economy；嗰時啲人俾經濟學個名）呢個領域定義做「對國家財富嘅本質同起因嘅探討」（an inquiry into the nature and causes of the wealth of nations）噉解，詳細啲係：

「

英文原文："a branch of the science of a statesman or legislator [with the twofold objectives of providing] a plentiful revenue or subsistence for the people ... [and] to supply the state or common-wealth with a revenue for the public services."^[12]
粵文翻譯：（政治經濟學係）一門政治家或者立法者嘅科學分枝，（兩個目標係提供）充份嘅收入或者生活所需俾人民... 同埋供應收入俾個國家或者聯邦等佢做到公共服務。

」

但係亞當史密夫嘅見解唔完全受到當代嘅學者接納。喺 1803 年，法國經濟學家 Jean-Baptiste Say 嘗試將呢個領域由佢喺公共政策上嘅用途區分開，將個領域定義做「研究財富嘅生產、分配、同消費嘅科學」^[13]，而再後嚟少少嘅英國哲學家米爾（John Stuart Mill）喺 1844 年嘗試響一個社會層面定義經濟學：

「

英文原文："The science which traces the laws of such of the phenomena of society as arise from the combined operations of mankind for the production of wealth, in so far as those phenomena are not modified by the pursuit of any other object."^[14]
粵文翻譯：科學嘅一門，專係探究由人類生產財富嘅共同作業所構成嘅社會現象當中嘅法則，只要呢啲現象冇受（人類）對任何第啲嘢嘅追求干擾。

」

打後喺 1890 年，經濟學家 Alfred Marshall 喺佢嗰本教科書《經濟學原則》（Principles of Economics）入面提出咗一個到咗家吓仲有好多人用嘅定義，將經濟學嘅分析擴展到唔淨指包含財富，以及由社會去到微觀經濟層面：

「

英文原文："Economics is a study of man in the ordinary business of life. It enquires how he gets his income and how he uses it. Thus, it is on the one side, the study of wealth and on the other and more important side, a part of the study of man."^[15]
粵文翻譯：經濟係喺生活嘅普通事宜嗰度對人嘅研究。佢（經濟學）探討個人點樣得到佢嘅收入同埋佢點樣用佢嘅收入。所以，佢（經濟學）一方面係對財富嘅研究，而喺另一個（更加緊要嘅）方面，係對人嘅研究嘅一部份。

」

廿世紀嘅定義

而喺 1932 年，英國經濟學家里奧勞·羅賓斯（Lionel Robbins）諗出咗一個俾人譽為「可能係而家最廣受人接納」嘅定義^[11]^[16]^[17]：

「

英文原文："Economics is a science which studies human behaviour as a relationship between ends and scarce means which have alternative uses."^[18]
粵文翻譯：經濟學係一門科學，專門研究人類行為，視人類行為係目的（end）以及稀缺得嚟又有多種用途嘅手段（mean）之間嘅關係。

」

羅賓斯形容話呢個定義唔係分類性噉將某啲類型嘅行為揀出嚟話係經濟學嘅研究對象，而係分析性噉將每種行為嘅一部份揀出嚟研究－即係受到稀缺性所主宰嗰部份^[19]。佢話佢之前嘅經濟學家通常都將研究嘅重點擺喺對財富嘅分析上，研究財富點樣俾人創造（生產）、分配、同消費，以及財富點樣增長^[20]，但係佢認為經濟學仲有得攞嚟研究第啲嘢，好似係打仗噉，因為打仗嘅各方通常都有一個共同嘅目的（要打贏）、涉及到成本同得益、而且會喺達到目的嘅過程當中會用資源（人命同財產等嘅損失）。如果喺一個局勢當中雙方都冇能力打贏或者成本高過得益好多嘅話，噉做決定嘅人（假設佢哋係理性嘅）永遠都唔會打仗（一個決定），而會嘗試用佢哋嘅資源去做第啲嘢。羅賓斯主張學界唔應該將經濟學界定為研究財富、戰爭、犯罪、教育、或者是但一樣可以用經濟學研究嘅嘢嘅科學領域，而係應該將經濟學界定為一個專門研究呢啲現象嘅某一樣共同部份－用有限嘅資源達到一啲目的－嘅領域^[6]^[21]。不過要留意嘅係，雖然羅賓斯呢個定義喺經濟學界廣受接納，但都唔係冇人批評^[16]^[22]^[23]。

稀缺性

稀缺性（scarcity）係經濟學最基礎嘅一個概念，佢指嘅係有限嘅資源同理論上無限嘅慾望（want）之間嘅差異：人類有好多慾望，但係就算有咗現代科技，佢哋手頭上嘅資源－錢、時間、精神－都硬係有限嘅，好多時佢哋都冇足夠嘅資源達到嗮佢哋想要達到嘅目的，例如一個人唔夠錢買嗮佢想要嘅嘢，唔夠時間做嗮佢想要做嘅嘢，又或者一間公司唔夠資源生產嗮佢想要生產嘅嘢。因為稀缺性嘅問題，人類需要做犧牲，需要放棄某啲嘢，嚟獲得更多佢想要嘅稀缺（scarce）嘅資源，亦都因為噉樣而有咗競爭，人同人之間會各自犧牲時間精神等嘅資源嚟爭稀缺嘅嘢。舉個例子說明，「人工高嘅工」係一種稀缺性嘅資源：好多人都想要賺多啲錢，所以會犧牲時間精神返工同搵人工高嘅工，而高人工嘅職位嘅數量少過想要呢啲職位嘅人嘅數量，所以啲人就要爭，佢哋會有誘因落力，犧牲更多時間同精神為求爭到嗰啲職位^[24]^{:p. 105}。

世上有一啲資源係冇稀缺性嘅問題嘅，好似係空氣噉就係一個例子，空氣對於地球上嘅生命嚟講不可或缺，但係佢對人類嚟講唔具有稀缺性（雖然對於某啲生物嚟講可能有）：一個正常嘅人類無時無刻一唞就唞到氣，唔使用啲乜嘢時間精神嚟令到自己有氣唞，而且目前地球上嘅空氣充足到不得了，夠嗮人類享用，所以人類根本冇誘因爭空氣－空氣呢種資源嘅量足夠滿足人對佢嘅需求，所以唔具有稀缺性^[25]^[26]。

稀缺性係經濟學嘅最根本概念。有好多資源都係稀缺嘅，所以人類要做好多決策，決定點樣運用某啲資源（時間、精神等等）去最大噉滿足自己嘅慾望：一個人會想用自己有限嘅收入滿足最多嘅慾望，好多時又會想要有多啲錢（等自己可以滿足多啲慾望），但係錢係有限嘅，所以人同人之間會爭錢、爭人工高嘅工；一間牟利公司會想用自己有限嘅人力賺最多嘅盈利，又會想要有多啲人力嚟賺多啲盈利，但係優秀人才嘅數量係有限嘅，所以公司同公司之間會爭人才；一個國家會想用自己有限嘅資源（自然資源同土地等）令自己最穩固，又會想要有多啲資源，而世上嘅資源係有限嘅，所以國家同國家會以外交甚至打仗嘅方式爭資源爭土地。呢啲競爭嘅過程迫使決策者要以最有效率嘅方式運用自己手上嘅資源－而「人類點樣運用手上資源，最有效率噉達到目的」正正就係經濟學研究嘅重心^[24]^[27]。

研究方法

當代經濟學係一門科學領域，目的係增加人類對經濟現象（稀缺性引起嘅現象）嘅了解。經濟學家會運用數學上嘅知識，利用微積分、線性代數、統計學、博弈論、同電腦科學等領域嘅方法分析同模擬經濟現象。一個專業嘅經濟學家要或多或少噉識得用呢啲架生同埋了解當中嘅原理，而有啲經濟學家仲會專門研究呢啲架生點樣喺經濟學上應用^[28]。

理論

主流經濟理論靠嘅係量化嘅經濟模型（economic model）。一個經濟模型會有若干個假設，並且由呢啲假設同已知嘅經濟學知識嗰度推導一個數學模型出嚟，目標係要用最少量嘅假設，做出最精確嘅預測同埋解釋最多嘅現象（即係科學哲學上所講嘅奧坎剃刀，第啲科學領域都會用呢條原則）。廿一世紀經濟學界主流嘅新古典經濟理論都跟實呢套準則，以建立能夠準確噉預測經濟現象嘅模型作為目標，令經濟學成為一門受人尊重嘅科學領域^[29]。

舉個簡單例子說明，以下係一個模擬一間公司嘅利潤嘅經濟模型^[30]：

\pi (x,t)=xp(x)-C(x)-tx\quad

當中 $\pi (x,t)$ 係指利潤， $x$ 係間公司供應貨品服務嘅率， $p(x)$ 係指喺 $x$ 之下啲貨品服務嘅價格， $C(x)$ 係指以 $x$ 呢個率供應貨品服務嘅成本，而 $t$ 就係指每賣一件貨品服務要交嘅稅。如果間公司想令自己利潤最大化，噉 $x$ 嘅理想數值可以用以下嘅方式計出：

{\frac {\partial \pi (x,t)}{\partial x}}={\frac {\partial (xp(x)-C(x))}{\partial x}}-t=0

；喺

\pi (x,t)

最大化嘅情況下，呢條式會成立（有關呢啲數學符號嘅意思，可以睇微積分）。

因為 $t$ 同 $C(x)$ 等有已知數值，所以用呢啲式可以計出 $x$ 嘅理想數值。

驗證

經濟學會嘗試用實證方法（empirical method）驗證佢啲理論。舉個具體啲嘅例子說明，喺貨幣理論（monetary theory）當中，貨幣量說（quantity theory of money）呢個理論模型入面嘅方程式預測錢供應嘅增長率上升會提高通脹，而通脹係假設咗受理性預期影響嘅，經濟學家跟手就會喺現實世界觀察呢啲現象，而如果佢哋嘅觀察數據合乎個理論嘅預測嘅話，就會加強咗佢哋對個理論模型嘅信心。有陣時，呢啲理論模型仲會做得到量化（quantitative）嘅預測－即係唔淨只預測到變數之間成正相關定負相關，仲能夠靠觀察到嘅數據同手上嘅數學方程式，近乎完美噉預測到目標變數跟住落嚟嘅數值^[31]^[32]。

喺驗證經濟理論嘅過程當中，統計學（statistics）係好常用嘅技術。經濟學研究者會用統計學上嘅方法，由數據嗰度估計一個統計效果（effect）有幾強同埋有幾高統計顯著性（statistical significance）等嘅資訊。例如，如果家陣有個假說話某兩個變數係成正比嘅（頭先貨幣量說嘅假說係「錢供應嘅增長率升會提高通脹」），噉個研究者可以去攞數據，睇吓嗰兩個變數係咪真係成正比。如果搵到嘅結果同假說所講嘅一致，就會增強經濟學家對個理論嘅信心^[33]。

經濟測量

經濟測量學（econometrics）係經濟學同統計學嘅結合領域，旨在諗點樣用統計學方法分析啲描述經濟現象嘅數據，靠噉嚟搵出唔同經濟現象之間嘅關係^[34]。技術性啲噉講，經濟測量學做嘅嘢可以噉樣描述^[35]：

「

原版英文："... The quantitative analysis of actual economic phenomena based on the concurrent development of theory and observation, related by appropriate methods of inference."
粵文翻譯：（經濟測量學係）對實際經濟現象嘅量化分析，基於同期嘅理論發展同埋觀察，而呢啲嘢係用適合嘅推論方法連繫埋一齊嘅。

」

經濟測量學嘅基礎係迴歸分析（regression analysis）。迴歸分析係一類嘅統計模型技術，指一啲用嚟建立描述兩個或者以上唔同變數之間嘅關係嘅數學模型^[36]：喺統計學上，研究者好多時會想用一個變數嘅數值嚟預測第啲變數嘅數值；喺最簡單嗰種情況下，個統計模型會涉及兩個連續性（continuous）嘅變數，當中一個係自變數（independent variable；IV），而另一個就係應變數（dependent variable；DV），而個研究者會用個 IV 嘅數值嚟預測個 DV 嘅數值；對個研究者嚟講，一個可能嘅做法係搜集啲數據返嚟，用啲數據做迴歸分析，整個模型（即係畫條線）出嚟，個模型就能夠幫佢預測「當 IV 係呢個數值嗰陣，假設第啲因素不變，個 DV 嘅數值會傾向係幾多」^[37]^[38]。迴歸模型有以下呢啲^[39]：

DV=a+b(IV)

（線性迴歸；linear regression）；

DV=a+b(IV)+c(IV)^{2}

（多項式迴歸；polynomial regression）；

... 等等。原則上，如果有個方法可以由過往數據嗰度搵出 $a$ 同埋 $b$ 等參數嘅數值，第時就可以靠條式大致上用 IV 嘅值估計 DV 嘅值；而家想像 $DV$ 係失業率，而 $IV$ 係本地生產總值（GDP）增長率，用過去數據得出參數 $a$ 同 $b$ 嘅數值，噉啲人第時就有可能睇到 GDP 增長嚟估計失業率嘅數值^[34]^[35]。

經濟實驗

經濟學－尤其係宏觀經濟學－嘅實證研究多數係靠觀察（observation）嘅，好少可會好似自然科學噉做實驗。噉係因為做實驗涉及嘅係特登操控一啲變數，再睇吓有乜效果－呢樣嘢喺研究社會嘅社會科學當中好難做到，而且攞人嚟隨意操控好多時會引起道德上嘅爭議（唔似得自然科學噉，有得攞自己嘅研究對象去實驗室隨意搞）。一般嚟講，科學界都會覺得觀察冇受操控嘅實驗噉嚴謹，認為觀察好多時冇辦法驗證一啲講因果關係嘅假說。所以廿一世紀嘅經濟學家開始咗嘗試用多啲實驗方法做研究，尤其係喺研究個人層面現象嘅微觀經濟學研究當中比較容易做實驗－研究者大可以搵啲人返去實驗室嗰度做某啲實驗作業^[33]。

實驗經濟學（experimental economics）嘗試用實驗驗證經濟學理論，令經濟學同自然科學之間嘅差距縮細咗^[40]^[41]。舉個例子說明，行為經濟學（behavioral economics）理論當中有所謂嘅損失規避（loss aversion）現象，指人類零舍在意損失，所以（例如）損失 100 文引致嘅滿足感喪失數值上大過獲得 100 文引致嘅滿足感得益。喺一個展視損失規避嘅實驗當中，研究者可以去搵一柞受試者返嚟玩一啲買賣遊戲，途中問佢哋（例如）寧願賺多 10 文定係避免損失 10 文，結果多數係發現啲受試者通常都會揀後者－引證咗損失規避嘅假說^[42]。類似嘅實驗都係搵一啲受試者返個實驗室嗰度，再要佢哋做一啲經濟上嘅決策，幫經濟學家驗證微觀經濟學嘅理論。同類嘅研究好少可會用嚟驗證宏觀經濟學上嘅學說－因為宏觀經濟學所研究嘅現象閒閒地都係喺城市以至國家層面嘅，好難擺喺實驗室入面做^[43]^[44]，不過好多人都認為，實驗經濟學上嘅發展令到經濟學成為咗一門「真正嘅科學」^[45]^[46]。

微觀經濟學概念

生產

喺微觀經濟學入面，生產（production）係一個由輸入去輸出嘅過程：喺生產嘅過程當中，做生產嘅人會用某啲輸入（生產因素，包括原材料、精神、同時間等）製造出一啲可以滿足某啲人嘅慾望嘅貨品或者服務，從而創造經濟價值（economic value）；例如一個喺廣東菜餐廳入面炒緊餸嘅廚師噉樣，佢會花費一啲原材料－生嘅菜同肉等等嘅食材－同埋用自己嘅精神時間，煮出一碟碟可以滿足啲食客（想食靚餸）嘅慾望嘅產品；生產嘅過程又可以係整一啲服務，例如係一個髮型師噉，佢會用一啲架生－飛髮嘅用具－同埋自己嘅精神時間幫啲客人飛髮，啲客人唔會得到任何貨品，但係個髮型師做嘅服務滿足到佢哋（想自己啲頭髮靚）嘅慾望；有陣時，一個生產過程會涉及大量嘅人力物力，而且同時滿足好多人嘅慾望，例如係起路噉，包括工程師同建築工人在內嘅大量人員會用各種原材料－好似水泥同鋼筋呀噉－同大柞架生，再加自己嘅精神時間去整一個產品－一條人同車行得嘅路－出嚟，滿足周圍嘅市民（想要有條路可以行）嘅慾望^[47]。

報酬遞減

報酬遞減（diminishing returns）係生產過程當中嘅一種現象。根據邊際報酬遞減定律（law of diminishing marginal returns），假設第啲因素不變，係噉提高某一個生產因素嘅量普遍會引致整體產量嘅提升，但邊際報酬（marginal return / marginal product；指報酬上升嘅率，設報酬做 $r$ ，生產因素嘅量做 $F$ ，邊際報酬由 ${\Delta r}/{\Delta F}$ 反映）會係噉跌，即係話如果將「產量」（Y 軸）同「生產因素 A 嘅量」（X 軸）畫幅圖，假設第啲生產因素不變，條線嘅斜率會隨住生產因素 A 嘅量上升愈嚟愈負^[48]^[49]：

舉個例子說明：一間建築公司家吓要起一棟建築物，個地盤嗰度有一定量嘅器械、材料、同工人等嘅生產因素；假想而家間建築公司個老細增加工人嘅數量，但其他生產因素（器械同材料）嘅量保持不變；根據邊際報酬遞減定律，佢呢個做法會引致建築效率提升，但隨住工人數量增加，就會搞到（例如）啲器械唔夠用，所以就算再加多啲工人都冇咁幫到手提升生產效率－工人數量愈多，再增加工人所引致嘅建築效率提升幅度就會變得愈嚟愈細，請新工人變得愈得愈唔著數。邊際報酬遞減定律說明咗，喺生產嘅過程當中唔可以係噉增加某一個生產因素嘅量，而要整體噉考慮成個生產過程當中嘅所有因素^[48]。

機會成本

"There ain't no such thing as a free lunch." （「呢個世上冇免費嘅午餐。」^[50]）

生產必然會涉及機會成本（opportunity cost），即係話喺生產嘅過程當中做生產嘅人放棄咗冇用啲資源去做第啲嘢，而做一次生產嘅機會成本就係「做佢而放棄咗嘅選擇當中最有價值嗰個」。例如如果班工程師同建築工人用咗啲材料喺澳門嗰度起咗條路，佢哋就冇得用嗰啲材料喺珠海或者香港嗰度起路，而呢啲如果冇起澳門嗰條路就有得起嘅路當中最有價值嗰條就係「用啲材料喺澳門起路」嘅機會成本。機會成本唔淨只包含錢同財物^[51]^[52]：例如一個麵包師傅可以用揀用佢一個上晝嘅時間焗菠蘿包，又可以揀用嗰個上晝打 PSP，又可以揀用嗰個上晝寫返篇有關菠蘿包嘅維基百科文。如果佢揀咗用個上晝寫維基文，噉佢就放棄咗用啲時間焗菠蘿包或者打 PSP，而佢做呢個選擇嘅機會成本就係另外兩個選擇當中比較有價值嗰個－喺呢個例子入面，所涉及嘅資源就係時間^[53]。

	焗菠蘿包	打 PSP	寫維基文
價值（以主觀效益量度）	60	40	30
揀呢個選項嘅機會成本	40	60	60

機會成本係稀缺性嘅必然結果：可用嘅資源係稀缺嘅，所以做決策嘅人喺消耗佢資源做生產嘅時候要做取捨，如果佢決定咗用資源做其中一樣嘢，噉佢就冇得做其他嗰啲嘢，而其他嘢當中最有價值嗰樣就係機會成本。對機會成本嘅分析係經濟學上嘅重要一環，如果一個選擇嘅機會成本高過另一個嘅（假設所有選擇嘅價值都可以用某啲數值量度），後者會係一個比較有效率嘅資源運用方法－揀佢所放棄嘅價值會比較少^[25]^[26]。

生產可能性曲線

生產可能性曲線（production-possibility frontier，簡稱「PPF」）係一種用嚟表達稀缺性、成本、同效率嘅圖。假想而家有間好簡單嘅工廠，佢淨係曉生產「槍」（gun）同「牛油」（butter）兩種貨品，佢幅 PPF 條 Y 軸係「生產嘅槍嘅量」，而 X 軸係「生產嘅牛油嘅量」。呢間工廠手頭上嘅資源係有限嘅，如果佢想生產多啲槍，佢就要生產少啲牛油－即係話兩個量之間嘅關係必然係負，條線所包含嘅面積必然係有限嘅。喺 B、D、同 C 呢三點嗰度，間工廠都係用嗮全力去生產，喺 A 嗰點，間工廠冇用嗮全力生產，而 X 嗰點係間工廠冇能力達到嘅一個狀態^[54]。

現實世界嘅 PPF 實係一條斜率成負數嘅線，而呢一點起於稀缺性嘅問題：如果有能力，間工廠會有咁多生產咁多，但係因為稀缺性嘅問題，佢手頭上嘅資源唔夠滿足佢「有咁多生產咁多」嘅慾望，所以間工廠嘅管理人就要做取捨－如果佢用嗮全力嘅話，佢只能夠一係生產多啲槍，一係生產多啲牛油，冇得話兩者兼得^[55]。

間工廠喺 PPF 所處嘅位置反映佢嘅效率（efficiency）嘅一部份：如果佢產量係喺 A 嘅話，噉就表示佢仲有能力同時生產更多嘅槍同生產更多嘅牛油，反映咗佢冇用全力，處於一個冇效率嘅狀態（而原因可能係因為太多工人偷懶，或者工人唔識善用啲機械等等）。另一方面，就算間工廠處於 B、D、或者 C（用嗮全力生產），都唔一定等如佢效率好好，例如如果佢出盡全力生產咗 100 支槍同 100 嚿牛油，但係啲客想要嘅係 80 枝槍同 120 嚿牛油嘅話，佢就達唔到理想嘅柏里圖最適（Pareto optimality）狀態^[56]－即係做唔到有效率噉滿足啲客嘅慾望。「點樣提升效率」喺經濟學上係一個好緊要嘅課題，例如喺經濟衰退嗰陣嘅高失業率令到好多人力都冇做生產，係一種冇效率嘅狀態，亦都係一種好受關注嘅現象^[57]。

消費同投資

喺做咗生產之後，一個人手上會賺取咗一啲資源，跟住佢有兩大選擇：佢可以即刻用咗呢啲資源佢，即時噉滿足自己嘅慾望；佢又可以投放啲資源去做唔能夠即時滿足慾望、但能夠喺將來引起更大得益嘅活動；舉個例說明，想像家吓有個人做生產過程，得到一個蘋果，佢可以即刻食咗，滿足自己對嘢食嘅慾望，但佢又可以攞個果實去種，將來種樖蘋果樹出嚟，等自己將來多啲蘋果食－要喺「即時滿足慾望」同埋「追求將來更大噉滿足慾望」之間二揀一，前者就係經濟學廣義上嘅消費（consumption），後者就係廣義上嘅投資（investment）^[58]^[59]。

喺行為經濟學（behavioral economics）上，消費同投資可以用延遲折扣（delay discounting）嘅概念嚟思考：延遲折扣係指一樣事物主觀上嘅效益會隨住「得到樣嘢要等嘅時間」而下降；想像一個實驗：擺一隻老鼠喺個籠入面，個籠入面有兩個掣，撳第一個掣會即刻得到一粒嘢食，而撳第二個掣會喺 5 分鐘之後得到一粒一樣嘅嘢食（兩個掣喺撳咗一嘢之後 5 分鐘之內唔會有反應）；實驗結果通常係，隻老鼠肚餓嗰陣會走去撳第一個掣，但會對第二個掣不聞不問；另一方面，如果更改兩個掣嘅結構，變成撳第二個掣會喺 5 分鐘之後得到一嚿大嚿啲嘅嘢食嘅話，隻老鼠就比較有可能會對第二個掣有興趣－即係話喺隻老鼠心目中，一嚿嘢食嘅效用 $u$ 會隨住「得到佢要等嘅時間」（ $D$ ）而下降（不過「 $D$ 同 $u$ 之間嗰個函數呈乜形狀」係一個受爭議嘅課題）。

喺嚿嘢食大細不變嘅情況下，一個延遲折扣實驗嘅可能數據
	撳咗即刻有嘢食	撳咗 3 分鐘後有嘢食	撳咗 5 分鐘後有嘢食
隻老鼠撳嗰個掣嘅機會率	65%	25%	10%

延遲折扣指緊嘅就係呢樣嘢－喺一個人心目中，一樣嘢嘅有利度會因為「延遲」而打「折扣」，而當一個人得到資源，佢就要選擇即時消費或者用件資源做投資以求未來得到更大利益（但呢個利益嘅主觀價值會因為延遲折扣而打咗折）－反映咗延遲折扣^[60]^[61]。行為經濟學對延遲折扣嘅思考幫到手做某啲有關消費同投資嘅政策制定^[62]。

分工

理論同實證經濟研究表示咗，要達到高嘅效率，其中一種最好嘅做法係分工（division of labor）^[63]。考慮返上面嗰個生產槍同牛油嘅工廠嘅例子說明，想像家吓嗰間廠隔籬開咗另一間都係生產槍同牛油嘅廠，但係兩間廠條 PPF 嘅斜率唔同：如果 A 廠要生產多 10 嚿牛油就要放棄生產少 10 支槍，而 B 廠（例如因為啲工人嘅專長同 A 廠嘅唔同）要生產多 10 嚿牛油就需要放棄生產成 50 支槍－喺呢個情況下，B 廠生產牛油嘅機會成本會大過 A 廠嘅好多，所以 B 廠比起 A 廠會更加有誘因集中於生產槍；同時，A 廠相比之下會比較有誘因集中去生產牛油。即係話 A 廠喺生產牛油上有比較優勢（comparative advantage），而 B 廠就喺生產槍上有比較優勢。而如果 A 廠同 B 廠開始各自集中生產自己叻生產嘅貨品，就形成咗一個分工嘅局勢^[64]^[65]。

	A 廠	B 廠
生產多 10 嚿牛油嘅機會成本	10 支槍	50 支槍
全力生產牛油所得	500 嚿牛油	1,000 嚿牛油
全力生產牛油所失	500 支槍	5,000 支槍

雖然呢個例子就噉望落好似係 A 廠比較叻做生產噉，但係 B 廠依然可能係有絕對優勢（absolute advantage）嘅。舉個例子說明，如果兩間廠都用嗮全力生產牛油嘅話，（可能）A 廠會生產到 500 嚿牛油，而 B 廠會生產到 1,000 嚿牛油，但係喺全力生產牛油嘅過程當中，A 廠只係放棄咗 500 支槍，而 B 廠就會放棄咗成 5,000 支槍－B 廠集中生產牛油嘅機會成本高 A 廠嘅一大截，所以 B 廠雖然喺生產兩樣貨品上都叻過 A 廠，但佢依然比較有誘因集中生產槍^[64]。

人同人之間有專業知識同技能上嘅差異，所以每個人專長生產嘅嘢都唔同－一個經濟體就有咗分工嘅現象^[64]^[66]。

貿易

分工同比較優勢係貿易（trade）嘅起源：人同人之間以至國家同國家之間喺「擅長生產乜嘢」呢一點上都有差異：例如假想而家有兩個人，A 君比較叻煮嘢食，B 君比較叻整電腦，A 君會比較有誘因去開餐廳，而 B 君會比較有誘因去搵份做電腦技術員嘅工，B 君冇 A 君咁肯花時間煮嘢食，所以日日晏晝都去幫襯 A 君間餐廳，將佢做技術員賺到嘅錢俾 A 君嚟換取嘢食－兩個人之間形成咗一種互利嘅貿易關係；呢種現象喺國與國之間都會睇到，例如俄羅斯嘅國土石油含量多過日本嘅好多，所以比起日本更加有誘因發展石油產業，而當日本想要用石油嗰陣，就可能會問俄羅斯攞，但俄羅斯（定係是但一個產油國）正路唔會免費送石油俾日本，所以日本要搵返啲有價值嘅嘢（最直接嘅係錢）送俾俄羅斯，等俄羅斯有誘因俾石油佢－國際貿易就形成咗^[67]^[68]。

世界嘅唔同地區之間年年都有大量貿易發生緊－地理同文化等方面嘅差異令到唔同地區擅長生產唔同嘅貨品同服務^[69]。

錢同貨幣

錢（money）係一樣源於貿易嘅需要而有嘅發明。攞返上面 A 君同 B 君嘅例子說明，B 君想食 A 君煮嘅飯餸，但 A 君要生活，唔可以吓吓都免費幫人煮飯餸，所以 B 君又要俾返啲有價值嘅嘢 A 君嚟換取飯餸。喺呢個情況下，B 君可以用以物易物（barter）嘅方法^[70]，例如係送啲電腦零件俾 A 君嚟換取飯餸。之但係，如果 A 君唔需要電腦零件嘅話，噉 B 君就會冇價值俾得到 A 君－單交易做唔成。而家想像有一樣嘢 X，佢係普遍有價值嘅，可以攞嚟換做任何嘅貨品或者服務都得，噉單交易就做得成－B 君可以俾某個量嘅 X 俾 A 君，等 A 君打後有得用佢開餐廳所賺返嚟嘅 X 去換取佢自己想要嘅嘢－而 X 就係錢。錢呢樣發明對於經濟嚟講好緊要，佢令到做起貿易上嚟簡單方便好多，而且仲扮演咗量度貨品服務嘅價值、令到人有得儲蓄、同容許延期付款等嘅多種功能^[71]^[72]。

廿一世紀世界嘅各個經濟體都會有喺自己內部流通嘅貨幣（currency），等佢內部嘅人有得做交易^[71]。


同港紙掛鉤嘅美元	歐羅	一張津巴布韋元銀紙；面額成 100 萬億。

喺經濟學上，一個政府應該點樣管理自己內部嘅貨幣係一個好緊要嘅課題。好似係通脹（指個經濟體內貨品服務嘅價格持續上脹）等嘅現象可以對民生造成巨大嘅衝擊。一個好經典嘅例子係津巴布韋元（Zimbabwean dollar）^[73]：津巴布韋元曾經係非洲南部國家津巴布韋（Zimbabwe）嘅貨幣，但因為政府管理不善，搞到津巴布韋國內由 1990 年代尾開始就有極嚴重嘅通脹，啲錢變到愈嚟愈冇價值，而啲貨品同服務嘅價錢係噉升，搞到政府要係噉印大啲面額嘅銀紙，打後仲出咗驚世嘅面額 100 萬億文嘅銀紙。最後喺 2009 年 4 月，津巴布韋政府索性停用津巴布韋元改用美元做貨幣，單嘢先至收到科。自從嗰陣之後，津巴布韋元喺經濟學界間唔中就俾人攞嚟做「失敗貨幣」嘅例子^[73]。

市場

當買家攞住啲錢，而賣家攞住佢哋覺得買家會有興趣買嘅貨品服務，雙方都準備好要做交易嗰陣，佢哋就需要喺某啲地方嗰度互相交流同埋做交易上嘅決定－呢啲空間就係所謂嘅市場（market）。市場可以係一啲實際上存在，睇得到摸得到嘅地方（好似係俾人買餸嘅街市噉），又可以係淨係存在喺互聯網當中嘅電子空間（好似係網上購物網站噉）。市場係買家同賣家溝通嘅主要媒介，亦都負責幫手俾雙方決定貨品服務嘅價格－買家同賣家會喺市場嘅空間當中接觸同交流，並且向對方傳達（例如）「呢件貨喺呢個價格我會買幾多幾多」等嘅資訊^[74]^[75]。


香港通菜街好多人喺度擺檔賣嘢。	廣州迎春花市有好多檔攤喺度賣花。	台灣嘅羅東夜市有好多檔攤。	投資者喺度用電子方式買賣股票。

自由市場

市場好多時都會受外界因素干預。理論上，如果一個市場係一個自由市場（free market；指淨係受消費者主導，唔受政府或者第啲權威干涉嘅市場），噉佢裏面嘅每件貨品同服務嘅價格遲早都會去到均衡嘅水平－即係價錢啱啱好令買家肯買嘅量同賣家肯生產嘅量一樣，令價格冇誘因改變。不過實際上，現實世界嘅市場會因為好多因素而去唔到均衡狀態，例如係政府可能會用稅等嘅措施人工噉令某啲貨品服務價格提升，雖然好多人都主張完全自由嘅市場先係好嘅，但係呢啲干預查實對民生嚟講好多時都係必要嘅：舉個例子說明，好多先進國家同地區（包括香港）嘅政府都會補貼內部嘅醫療服務，等啲老百姓有得用平啲嘅價錢（甚至乎係唔使俾錢）睇醫生；如果呢啲政府唔噉樣做，而係由得啲醫療服務嘅價格由自由市場話事嘅話，啲醫療服務就好可能會貴好多－會搞到好多老百姓嘅健康冇咗基本保障^[76]。

市場結構

市場可以分做好多種唔同嘅市場結構（market structure）^[77]：

喺一個具有完全競爭（perfect competition）嘅市場當中，冇任何一個買家或者賣家能力勁到能夠單方面噉控制一個產品嘅價格－每一個個體買家同賣家都有份講價，掌控市場嘅力能係平均噉分佈嘅。但係呢個好多時只不過係為咗理論分析上嘅方便而做嘅理想化（idealization）：喺現實世界當中，市場多數都係達唔到完全競爭呢樣特性嘅。
現實世界有啲市場係壟斷（monopoly）型嘅－即係話喺呢啲市場入面件貨品服務得一個賣家。仲又有所謂嘅雙佔（duopoly；指得兩個賣家）同埋寡頭壟斷（oligopoly；指得少數幾個賣家）。
買方壟斷（monopsony）係指個市場得嗰一個買家。
喺呢啲唔完全競爭嘅市場之下，掌控市場嘅力量係唔平均噉分佈嘅。舉個例子說明，喺壟斷或者雙佔等嘅局面之下，好多時嗰啲少數嘅賣家會為咗自己嘅利益而夾埋一齊用手段唔俾新嘅賣家加入個市場－等佢哋自己有得呼風喚雨，任意操控價格嚟將自己嘅利潤最大化。

供應同需求

喺一個市場入面，是但搵一樣俾人買賣緊嘅貨品或者服務，佢會至少有兩個屬性：價格（price）同埋數量（quantity）^[78]。而供應同需求（supply and demand）嘅理論就解釋價格同數量之間嘅啦掕。喺微觀經濟學上，呢個理論能夠預測喺一個有完全競爭（perfect competition；指冇任何一個買家或者賣家大到有能力獨力掌控啲貨品服務嘅價格）嘅市場下嘅貨品同服務嘅價格^[77]。

需求

喺一個市場之下，對一件貨品服務嘅需求（demand）指需求量（quantity demanded；買家有能力同肯買嘅量）同價格之間嘅關係。例如有件貨品服務，佢價格喺 10 文嗰陣啲買家總共會買 100 件，價格喺 9 文嗰陣啲買家總共會買 110 件... 等等嘅話，佢嗰條需求曲線（demand curve）就會係一條穿過（10 文，100 件）同（9 文，110 件）呢兩個點嘅線（唔一定係直線），而呢條曲線所表達嘅嗰段關係就係個需求^[79]。需求理論將個體消費者描述為理性嘅決策者，假設佢哋會嘗試用自己手上嘅有限資源（即係佢哋嘅收入）最大噉滿足自己嘅慾望－用行話講嘅話，就係喺有限嘅資源下將自身嘅效用（utility）最大化^[79]。

需求定律（law of demand）宣稱，是但搵個市場同是但一件貨品服務，假設第啲因素不變，佢價格同買家會買嘅量之間會形成負關係（條線嘅斜率 $s$ 係負數）－價格愈高買家就會買得愈少，而價格愈低買家就會買得愈多。呢條定律係稀缺性嘅必然結果：價格係啲買家每買一件就要付出嘅資源量，所以一件貨品服務嘅價格乘需求量（買家有能力又肯買嘅量）反映買家有能力付出幾多資源嚟買件貨品服務，但錢係一樣有限嘅資源，所以價格 $P$ 同需求量 $Q$ 乘埋出嗰個數（ $P\times Q$ ）一定係一個有限嘅數值；如果條需求曲線係由頭到尾都打橫（ $s=0$ ）、打直（ $s=\infty$ ）或者成正斜率（ $s>0$ ）嘅話，就會出現價格同需求量乘埋個數可以係無限大嘅情況，所以如果錢有稀缺性，噉需求曲線只可以係一條斜率係負數嘅線^[80]。

某啲外界因素可以改變一個需求：收入可以影響需求－如果啲買家嘅收入突然間普遍升嘅話，噉通常會引致條需求曲線向右移（買家無論價格幾高都買多咗嘢），而相反，如果啲買家收入突然普遍跌嘅話，噉通常會引致條需求曲線向左移（買家無論價格幾高都買少咗嘢）^[81]；替代品（substitute）嘅價格都可以改變需求，例如如果家吓 SONY 提高佢啲商品嘅價格，搞到由佢生產嘅 PS4 遊戲機價格上升，噉啲買家對任天堂 Switch（由任天堂出嘅遊戲機）嘅需求會提升（條需求曲線向右移）－PS4 價格上升令少咗人買 PS4，而任天堂 Switch 同 PS4 一樣，可以滿足啲人打機嘅慾望（係 PS4 嘅替代品），所以就算任天堂 Switch 嘅價格唔變都好，啲人都會買多咗^{[註 1]}^{[註 2]}^[82]。

喺現實應用當中，經濟學家會用各式各樣嘅方法嚟做需求預測（demand forecasting）：喺最簡單嗰種情況下，佢哋可以做個問卷調查，派問卷俾啲消費者，問佢哋「如果件貨品服務係呢個價錢，你會買幾多件」等嘅問題，再將消費者嘅答覆畫成圖表（Y 軸係價格，X 軸係需求量），跟住做個迴歸分析（regression analysis；簡單啲講就係一種分析方法，能夠靠手上數據畫出反映兩個變數之間嘅關係嗰條線^[83]），結果會得出一條估計嘅需求曲線。需求預測嘅技術喺營銷策略上好有用，可以幫到企業家知道喺每個價格件貨品服務會賣到幾多件，並且靠噉估計件貨品服務要設喺乜嘢價格先可以令利潤最大化^[84]。

供應

供應（supply）係指件貨品服務嘅價格同供應量（quantity supplied；指賣家肯整嚟賣嘅量）之間嘅關係。例如假想一件貨，佢價格喺 10 文嗰陣啲生產商總共會生產 100 件，價格喺 9 文嗰陣啲生產商總共會生產 90 件... 等等嘅話，噉呢個價格同供應量之間嘅關係就係件貨嘅供應。對生產者嚟講，佢哋賣一樣貨品服務嘅所得等如件貨品服務嘅價格乘以所賣嘅量，所以一般嚟講，如果假設生產者係理性噉想自己嘅所得最大化嘅話，佢哋會嘗試生產同供應令到佢哋利潤最大化嘅貨品服務，而噉通常就示，假設第啲因素（例如生產成本）不變，一件貨品服務嘅價格愈高，啲生產者賣佢就愈會賺到錢，就愈會肯生產佢－是但搵條供應曲線（supply curve）嚟睇，佢嘅斜率都會係正數，呢條就係所謂嘅供應定律（law of supply）^[85]。

同需求同一道理，外在嘅因素可以影響一個供應：例如如果生產科技上有突破或者工人嘅人工跌，令一件貨嘅生產成本下降嘅話，噉通常條供應曲線會向右移（就算價錢不變，啲生產商都會生產多咗）；而如果工人嘅人工升，搞到生產成本上升嘅話，噉通常條供應曲線會向左移（就算價錢不變，啲生產商都會生產少咗）^[85]。

市場均衡

市場均衡（market equilibrium）係指供應量同需求量相等嘅一種狀態，即係喺幅供應需求圖入面供應曲線同需求曲線相交嗰一點。

當一件貨品服務嘅價格低過佢嘅均衡點嗰陣，需求量會大過供應量，所以會出現短缺（shortage）；
而當件貨品服務嘅價格高過均衡點嗰陣，供應量會大過需求量，所以會出現盈餘（surplus）。

喺短缺發生嗰陣，通常啲買家就會爭貨，鬥俾高價錢買－就會推高個價，而喺盈餘發生嗰陣，啲賣家就要賤買手上嘅存貨－就會撳低個價。所以供應同需求理論亦都預測咗，長遠嚟講，任何一個貨品服務如果外在因素不變，佢個價格係會慢慢噉趨向均衡狀態，令到個供應量同需求量最後變成相等嘅，而當有某啲外在因素改變搞到成條供應曲線或者成條需求曲線有所改變嗰陣，個均衡價格會跟住變^[86]。

彈性

彈性（elasticity）喺經濟學上係需求同供應可以有嘅一種屬性，指需求量同供應量對價格改變幾有反應^[87]。需求同供應彈性可以用以下呢條式計：

e_{\langle p\rangle }={\frac {\mathrm {d} Q/Q}{\mathrm {d} P/P}}

；

當中 $P$ 係件貨品服務嘅價格， $Q$ 係件貨品服務嘅需求量（如果考慮緊嘅係需求）或者供應量（如果考慮緊嘅係供應），而 $\mathrm {d}$ 係指導數－簡單啲講就係變率^[87]。

舉個例說明，假想家陣有兩件貨品，X 同 Y，兩個條需求曲線都成直線，X 嗰條斜率高啲（需求彈性低啲），噉嘅話如果兩件貨品嘅價格同時上升 1 文，對 X 嘅需求量嘅跌幅會細過對 Y 嘅需求量嘅。即係話 X 嘅需求量對價格改變反應冇咁大－就算 X 價格變，消費者買嘅數量都冇變得咁明顯。X 需求彈性低啲可以係因為好多原因：例如可能 X 係一樣好似鹽同糖噉嘅生活必需品，所以就算 X 平咗，消費者都唔會太有動力買多啲，而 X 就算貴咗，好多消費者都仲係要焗住買咁多；Y 就可能係一啲好似魚子醬噉嘅奢侈品，Y 對消費者嚟講唔係生活必需嘅，所以佢價格一升，消費者可以索性唔買，但 Y 一平，就會好多消費者想買^[87]。

市場失靈

市場失靈（market failure）指一啲可能令個市場達唔到理想狀態嘅情況，包括咗以下呢啲^[88]：

資訊不對稱（information asymmetry）指個市場裏面某啲人手上嘅資訊明顯豐富過其他人手上嘅，例如喺一個股票市場入面，如果某個賣家知嘅資訊多過某啲買家，佢就可能會為咗自己嘅利益而呃啲買家買佢手上一隻佢知會跌嘅股票。呢種情況會搞到啲買家本嚟可以投資落去有建設性嘅嘢度嘅錢冇咗，造成市場達唔到最高嘅效率^[89]。
自然壟斷（natural monopoly）係一種特殊情況，因為加入一個產業需要龐大嘅成本同第啲阻止外界加入生產嘅因素，搞到個市場得一個賣家，而個賣家唔使同第啲賣家競爭之餘，仲可以單方面操控價格（相比之下，如果個市場有多個賣家，噉一個賣家加價之前起碼要考慮自己會唔會因為競爭對手唔加價而冇咗啲客），造成市場達唔到最高嘅效率^[90]。
共用品（public goods）指社會大眾普遍都有得使用嘅貨品服務，例如係街燈噉。呢啲貨品服務好多時係唔使俾錢都可以用到嘅（即係話價格好低），好少可會有政府以外嘅人自願噉供應佢哋，所以呢啲貨品服務好多時都會有短缺嘅情況。

不確定性

喺經濟學嘅分析上，當人嘗試將自己利益最大化嗰陣，過程當中往往有啲不確定性（uncertainty）喺度：一個人喺揀股票買嗰時，佢或多或少都會有啲唔肯定隻股票係咪真係會升；一個家庭喺選擇自己點使自己啲錢嗰陣，要考慮未來嘅收入可能會因為人工上落而有變動（所以要儲定啲錢以備不時之需）。而成個通訊行業嘅存在就係建基於不確定性嘅^[91]^[92]。

某啲市場組織嘅存在可能會引致不確定性，因而降低市場效率。例如美國經濟學家佐治·阿卡諾夫（George Akerlof）曾經做過分析，用二手車市場做例子：佢指出好多時啲買家都唔係好識睇車，所以唔知道一架二手車真正嘅價值係幾多（有不確定性），而啲賣家相比之下通常都好熟車方面嘅嘢嘅－即係話有一個資訊不對稱嘅情況^[93]；喺呢個情況下，啲賣家好多時會利用呢一點嚟呃呃氹氹啲買家，等佢哋用過高嘅價格買一啲實際上唔值咁多錢嘅二手車，搞到個市場入面嘅錢唔能夠完全有效率噉俾人使用。同類嘅情況喺保險業嗰度都有發生^[94]。

資訊經濟學（information economics）專門研究資訊喺經濟過程當中所扮演嘅角色－資訊係不確定性嘅相對，一個人手頭上有愈多資訊，佢嘅決策就愈少不確定性喺入面。資訊經濟學喺保險、合約、機制設計、同貨幣經濟學等嘅領域上都有應用價值^[91]^[94]。

成本效益分析

成本效益分析（cost-benefit analysis，簡稱「CBA」）係經濟學理論嘅一種常見應用，指一個人或者組織做決策嗰陣有系統性噉考慮嗮所有可能方案，並且逐個逐個方案諗吓個方案嘅利弊，最後按照分析結果決定到底應該採取邊個方案^[95]^[96]。運用經濟學嚟做成本效益分析嘅過程大致上如下^[95]：

搜集相關嘅數據，例如係間公司手頭上嘅財政資源同埋每一個方案嘅成本等等^[97]；
按照數據同經濟學理論（以及第啲相關嘅知識），列出每一個方案嘅優點同缺點；
按照數據同經濟學理論（以及第啲相關嘅知識），估計每一個優點會造成嘅得益同每一個缺點會造成嘅損失，得益同損失要用某啲數字量度，喺實際應用上，得失多數係用錢嚟量度嘅；
按照呢啲資訊，估計每一個方案嘅投資回報率（return on investment；ROI，指純利同成本所成嘅比例）；
選擇對自己最有利（投資回報率最高）嘅方案。

舉個簡單嘅例子說明：家陣有一間工廠嘅老細，佢喺度諗緊好唔好買機械人返嚟幫手做嘢（方案）。按照佢對機械人學同工業生產等領域嘅知識，佢知道每部機械人可以做相當於 5 個人類工人嘅嘢，並且作出以下嘅分析（搜捕數據，再按照數據同相關知識，估計每個方案嘅得失）：間廠目前有 20 個工人，每個工人嘅人工係 20,000 文一個月，所以每個月間廠要使 400,000 文喺請工人嗰度，而如果唔使諗通脹或者人工上漲等嘅現象嘅話，間廠喺未來 10 年要使大約 48,000,000 文嚟請工人（方案 1 嘅得失）；相比之下，如果間廠要全面自動化，就要買 4 部機械人，買一部機械人要使 1,000,000 文，呢個價錢包括埋維修嘅費用，而且用機械人嘅話，佢間廠要請個機械人專家幫手，而個機械人專家每個月嘅人工係 50,000 文，請佢請足 10 年就要使 6,000,000 文，所以將間廠全面自動化嘅成本 = 4,000,000 文 + 6,000,000 文 = 10,000,000 文（方案 2 嘅得失）。假如全面自動化同用工人所得嘅營業額一樣嘅話，噉對個老細嚟講，佢會比較有誘因選擇全面自動化（選擇對自己最有利嘅方案）^[98]。

博弈論

博弈論（game theory）係經濟學常用嘅一種數學理論，專門研究曉做決策嘅個體點樣喺策略上互動：博弈論提供咗一個數學基礎俾工業組織，等佢哋可以用數學模型模擬公司點樣做決策嘗試令自己嘅利益最大化，做買賣、同員工傾人工、講數、同設計合約等嘅過程都可以用博弈論嚟模擬^[99]^[100]。

舉個簡單嘅例子說明，想像家吓有兩間公司，叫公司 1（firm 1）同公司 2（firm 2），喺度鬥爭一個市場嘅買家，每間公司都有兩個選擇，一係揀「進入市場」（entry，簡稱「E」），一係揀「遲啲先進入市場」（delay entry，簡稱「DE」）。喺呢個個案之下，如果兩間同時進入市場嘅話，佢哋都會各自損失 4,000,000 文嘅盈利；而如果公司 1 而家進入市場，而公司 2 揀遲啲先入，噉公司 1 會得到 6,000,000 文嘅盈利，而公司 2 乜嘢都得唔到；淨低嘅情況可以睇圖 1。喺呢個形勢之下，對於公司 1 嚟講，揀 DE 嘅話實蝕，相比之下，佢揀 E 嘅話至少會蝕少啲，甚至有機會賺（如果公司 2 揀 DE 嘅話），同時，就算公司 2 知道呢點，佢可能都會因為唔想蝕 4,000,000 而焗住揀 DE。如果係噉，最後結果會係公司 1 賺 6,000,000，而公司 2 冇得益冇損失。有咗呢個模型模擬兩個公司之間嘅交鋒，一個經濟學家跟手可以模擬（例如）成個經濟體內部嘅眾公司彼此之間嘅交鋒，再睇吓（假設啲公司冚唪唥都係會揀令自己盈利最大化嘅選擇）佢哋最後邊個會係贏家。

同樣嘅數學模型仲有得攞嚟模擬個人層面或者國家層面嘅博弈^[101]^[102]。

宏觀經濟學概念

顯示中國大陸各地 2017 年 GDP 嘅地圖；數據顯示，廣東嘅 GDP 喺 2017 年全中國大陸第一。

商業週期嘅圖解；幅圖 Y 軸係（例如）GDP，而 X 軸係時間。

美國 1995 年至 2013 年嘅失業率數據

宏觀經濟學（macroeconomics）係嘗試整體噉研究一個經濟體嘅經濟學分枝^[103]^[104]，特點係會用一啲合計性（aggregate）嘅變數做研究嘅中心－即係將個經濟體入面一大柞個體嘅屬性合埋一齊，做一個整體屬性，再研究呢個整體屬性^[105]。舉個例說明，本地生產總值（gross domestic product；簡稱「GDP」）係宏觀經濟學成日研究嘅一個變數：將某段時間之內一個經濟體裏面嘅最終貨品服務嘅市場價值冚唪唥加埋一齊，得出個數就係嗰個經濟體喺嗰段時間之內嘅 GDP。喺計 GDP 嘅過程之中，個分析者將個經濟體入面一大柞個體物件（貨品服務）嘅屬性（市場價值）以某啲方法結合埋做一個數值（就噉加埋一齊），所以 GDP 呢個課題屬於宏觀經濟學研究嘅範疇^[105]。宏觀經濟學亦都會研究一啲影響成個經濟體嘅嘢，好似係個政府嘅政策噉^[105]。

喺建立經濟模型嗰陣，微觀經濟學嗰柞概念會俾人用嚟建構宏觀經濟學嘅理論：微觀經濟學研究嘅係一啲個人同公司層面嘅現象，而隨住經濟學家加深理解呢啲嘢點樣運作，佢哋可以用一啲相對簡單嘅數學模型或者方程式表達呢啲嘢嘅運作定律出嚟，再用呢啲定律模擬吓，睇吓（例如）如果個經濟體入面 50% 嘅個體都跟從嗰條定律行事嘅話個經濟體嘅 GDP 會變成點，再睇吓如果個經濟體入面 70% 嘅個體都跟從嗰條定律行事嘅話個 GDP 又會點^[106]^[107]。

本地生產總值

本地生產總值（gross domestic product；簡稱「GDP」）係一種常用嘅經濟指標（economic indicator），用嚟量度一個經濟體（「一個經濟體」可以係指一個城市、一個省、以至一個國家）喺某一段時間之間嘅經濟產出。計法如下^[108]^[109]：

GDP = C + I + G + (X − M)；喺呢條式當中，

C 代表消費（consumption），指個經濟體當中啲人使咗幾多錢買一啲最終貨品同服務（「最終」即係話件貨品服務唔會俾人再攞嚟做第啲生產），通常都係 GDP 入面最大嚿嗰個成份。
I 代表投資（investment），包括個經濟體入面嗰啲公司用咗幾多錢嚟買啲生產因素，但唔包現存財物或者金融產品嘅購買。
G 代表政府使嘅錢（government spending），指個政府使咗幾多錢買一啲最終貨品同服務，包括咗佢哋使嚟請工務人員、幫軍隊買武器、同投資嘅錢呀噉。
X 代表出口（export），指佢賣咗幾多貨品服務俾第啲經濟體消費。
M 代表進口（import），指佢由第啲經濟體嗰度買咗幾多貨品服務做消費。呢個數字係要由 GDP 嗰度減出嚟嘅，噉係因為個經濟體嘅消費（C 同 G 等）當中梗會有部份包括咗進口貨品服務嘅消費（例如一個平民買來佬貨），而呢啲進口貨品服務唔應該屬於本地生產嘅價值，所以要由 GDP 嗰度減返出嚟^[108]。

除咗 GDP 之外，經濟學界仲有第啲指標用嚟量度經濟表現，而比較唔同指標俾嘅結果能夠提供一啲有用嘅資訊。舉個例說明，人均本地生產總值（GDP per capita）係指一個經濟體嘅 GDP 除以人口，而喺 2018 年，香港雖然 GDP 總值低美國一大截，但人均 GDP 就拍得住美國^[110]，表示香港喺生產水準上拍得住美國，但因為人口細一截而令到總體經濟規模唔及美國－比較兩地嘅 GDP 同人均 GDP 幫手推導出一啲有關兩地經濟規模同概況嘅資訊。事實係，喺做經濟分析嗰陣，經濟專家通常會睇嗮唔同嘅經濟表現指標所俾嘅結果，靠噉嚟到判斷一個經濟體內部發生緊啲乜嘢事^[111]^[112]。

經濟增長

經濟增長（economic growth）指一個經濟體嘅人均產出（以按通脹調整後嘅 GDP 等嘅經濟指標計算）隨時間嘅提升^[105]。經濟增長可以受好多因素影響，包括咗成日有經濟學家研究嘅投資率、人口增長、同埋科技上嘅改變呀噉，例如中國喺廿一世紀初嘅驚人經濟增長噉，一部份就係因為內陸啲鄉下地區嘅人紛紛走去大城市打工，令工人嘅數量突然間升得好緊要（多咗好多人手做生產活動）而引起嘅^[113]。世界各地喺呢啲因素上嘅差異令到唔同嘅經濟體有唔同嘅經濟增長率，所以唔同嘅經濟體有啲會有錢啲有啲會窮啲^[114]。

商業週期

每一個經濟體嘅 GDP（同埋其他嘅經濟表現指標）長遠嚟講都會週期性噉上上落落，呢啲上上落落就係所謂嘅商業週期（business cycle）。一個商業週期包含一段相對蕭條嘅時期同一段相對興旺嘅時期^[115]。對商業週期嘅研究起源喺 1930 年代嘅大蕭條（The Great Depression）時期，當時英國經濟學家凱恩斯（John Maynard Keynes）寫咗佢嗰本名著《The General Theory of Employment, Interest and Money》（粵文：就業、利息同錢嘅廣義理論）嚟提出佢心目中嘅經濟理論，佢認為喺經濟不景嗰陣，對貨品服務嘅需求唔夠，搞到失業率高企同埋可能輸出嘅喪失。佢跟手仲主張要有一啲政策，例如係中央銀行嘅貨幣政策行動或者政府嘅財政政策嚟令到商業週期當中嘅輸出穩定化。凱恩斯嘅理論亦都表明咗，喺某啲情況下，一個經濟體冇乜嘢有效又自動性嘅機制能夠將個經濟體移向全民就業。呢啲文獻最後創出咗商業週期嘅概念^[116]^[117]。

失業率

一個經濟體入面嘅失業情況係用失業率（unemployment rate）量度嘅。失業率係描述個經濟體嘅勞動力（labor force）當中有幾多工人係冇工返嘅。勞動力包括咗主動噉搵緊工嘅工人－退咗休、仲讀緊書、或者因為個市場冇職位所以冇心機搵工嘅人都唔包括喺勞動力之內。失業嘅成因可以有好多：最簡單嘅情況係個經濟體入面職位唔夠^[118]，喺古典嘅經濟學理論當中，失業發生嘅主因在於人工高得滯，搞到啲老細冇誘因請多啲工人（即係工人供過於求），而人工高得滯嘅原因可能係因為最低工資法律或者工會嘅活動，而就算個經濟體冇工人供應量多得滯嘅問題，一個工人依然可能會經歷摩擦性失業（frictional unemployment）－例如一個工人辭職去搵第份工，就算個經濟體冇職位太少嘅問題，佢喺開始搵工到搵到嗰段時間都算係處於暫時失業嘅狀態^[118]。

結構性失業（structural unemployment）包括咗一大柞可能引致失業嘅原因，例如係工人嘅技能同現存職位所需嘅技能唔夾噉，呢種情況可能會喺個經濟體轉型嗰陣時發生^[119]。結構性失業同摩擦性失業相似嘅係，兩者都反映咗將工人安配落去職位空缺上有問題，而結構性失業反映咗工人需要時間學新技能嚟適應新嘅經濟環境^[120]。

無論個經濟體情況係點，佢當中梗會或多或少噉發生失業嘅情況。當一個經濟體嘅增長放緩嗰陣時，好多時會發生週期性失業（cyclical unemployment）嘅情況。奧肯法則（Okun's law）代表咗喺實證研究上搵到嘅失業率同經濟增長之間嘅關係^[121]，原版嘅奧肯法則話，經濟輸出每升 3% 失業率就會跌 1%。打後嘅研究有按照實證觀察調較條定律入面嗰兩個數值^[122]。

經濟政策

貨幣政策

喺廿一世紀嘅各大經濟體當中，錢都係交易嘅基礎，而啲貨品服務嘅價格基本上冚唪唥都係以「要幾多錢」嚟量度嘅。各大經濟體會攞襟用、易攜、供應有彈性、又唔易爛嘅物件嚟做錢－好似銀仔同銀紙等都符合呢啲條件^[123]。銀仔同銀紙本身並冇能力滿足人類嘅慾望，佢哋有價值只不過係因為個經濟體內部嘅人普偏都信佢哋有價值，令到啲人可以放心攞佢哋嚟買滿足到慾望嘅貨品服務。所以每一個經濟體都要做啲嘢，確保佢嘅貨幣係俾到信心人嘅，而管理得唔好嘅就會好似頭先提到嘅津巴布韋噉搞出禍。貨幣政策（monetary policy）指緊一個經濟體內嘅某啲權威性組織－即係中央銀行等－透過控制（例如）利息等嘅因素嚟到確保個經濟體內部嘅錢價值穩定同埋有公信力^[124]^[125]。

通脹（inflation）同通縮（deflation）係貨幣政策最常要應付嘅兩種情況，前者指個經濟體內部啲貨品服務嘅價格持續係噉升，而後者指個經濟體內部啲貨品服務嘅價格持續係噉跌，兩者都反映咗個經濟體嘅貨幣價值唔穩定：例如係通脹噉，想像有個人佢銀行戶口有 100,000 文，而喺個經濟體經過一個月嘅持續通脹之後，佢個銀行戶口入面錢嘅量冇變，但因為成個經濟體啲貨品服務嘅價格都升咗，佢攞住佢嗰 100,000 文實質上可以買到嘅嘢就少咗－佢啲錢嘅實際價值跌咗。無論通脹定通縮都會搞到啲錢嘅實際價值唔穩定，變相會損害啲人對隻貨幣嘅信心－而正如頭先提及，啲人對隻貨幣嘅信心係錢有價值嘅根本原因^[126]。

財政政策

財政政策（fiscal policy）涉及一個經濟體嘅政府用佢嘅收入同支出嚟影響個經濟體嘅宏觀情況：當一個經濟體嘅總需求跌到有返咁上下低嗰陣，就會出現有一部份生產能力冇用盡嘅情況；喺呢個情況下，政府可以使錢同減稅令個總需求升返，而用淨嘅資源可以俾政府攞去用。舉個例說明，想像家吓有一個經濟體經濟不景，搞到有好多唔同行業嘅人失業，喺呢個時候，政府可以減稅，等啲消費者多啲多餘錢，令佢哋使多啲錢，啲生產者感覺到消費者對貨品服務嘅需求上升，就會做多啲生產，就要請多啲人做嘢－減稅間接舒緩失業嘅問題；同時，政府又可以索性（例如）請一啲因為經濟不景而冇嘢做嘅建築工人同埋工程師幫政府手起基建－直接噉舒緩失業嘅問題^[127]。

公共事業

公共財政學（public finance）係涉及處理公共事業（public sector）嘅收支預算嘅經濟子領域，通常係處理返政府方面嘅嘢。呢個範疇內嘅事宜會含有比如稅收負擔（tax incidence，由邊個實際承擔個別嘅稅項）、分析政府主導啲項目嘅成效、唔同嘅稅收、同埋開支對經濟效率同收入分配嘅影響，仲有係經濟政策呀噉。後者係屬於公共選擇理論（public choice theory）嘅一部分，類似微觀經濟學噉研究公共事業啲活動，譬如（設想係）自私嘅選民、政客、同生意佬之間嘅啲互動^[128]。

多數經濟學科屬於實證經濟學，意圖預測同解讀啲經濟現象，而規範經濟學係以鑑證經濟表現理應係乜樣嚟做目標。福利經濟學（welfare economics）就屬於後種類型嘅分支經濟學科，佢會用微觀經濟學嘅理論技巧，去同時確定一個經濟體入邊嘅分配效率同埋相關嘅收入分配。呢個學科係會嘗試檢視構成咗社會嘅個體嘅經濟行為去研判社會福利^[129]。

理論史

人類早喺遠古時期經已有人寫經濟相關嘅著作，就連秦始皇都有執筆寫咗啲經濟方面嘅分析^[130]^[131]^[132]，但有系統性嘅經濟理論要去到成 16 世紀左右先開始出現：當時嘅歐洲人開始思考貿易同經濟政策方面嘅理論；好似重商主義（mercantilism）係 16 至 18 世紀嘅歐洲盛行嘅一套經濟政策理論，重商主義主張，一個國家嘅財富主要係由金同銀嘅累積主宰嘅，而領土唔夠金銀礦藏嘅國家只能夠靠同外國做買賣先可以得到金銀，因為噉，重商主義主張國家應該入口平價嘅物料嚟整貨品，並且用出口嘅方法嚟得到金銀^[133]；及後 18 世紀嘅法國思想家同作家諗出咗重農主義（physiocracy）嘅思想，將經濟體視為收入同產出嘅循環，相信只有農業能夠確實噉靠生產創造盈餘，所以指出農業係所有財富嘅基礎，因為噉，重農主義者反對重商主義者所提倡嗰種重視製造業同貿易嘅政策方針，亦都對稅收政策要點定先可以令財富最大化有所著墨^[134]^[135]。當時學者對呢兩個理論嘅辯論成為咗現代經濟學嘅起始，而亞當史密夫對重商主義嘅批判形成咗早期嘅經濟理論^[136]^[137]。

古典經濟學

喺 1776 年，亞當史密夫（Adam Smith）出版咗佢本名著《國富論》（The Wealth of Nations）。呢本書俾人譽為「造就經濟學誕生嘅起點」。喺呢本書入面，亞當史密夫指出土地、工作力、同資本係生產嘅三大因素，亦係一個國家嘅財富基礎，有別於主張「只有農業至係有生產力」嘅重農主義。而亞當史密夫同佢同期嘅學者所提倡嘅嗰柞諗法就係所謂嘅古典經濟學（classical economics）－呢套經濟學係現代經濟學嘅基礎，因為古典經濟學家提出咗好多現代經濟學仲用緊嘅概念^[138]^[139]。

亞當史密夫佢本人諗出咗好多為現代經濟奠基嘅諗頭：亞當史密夫指出，無論喺城市定係國家層面，分工都可以引致貿易同埋勞工生產力上升^[140]，當中佢個條定理「分工嘅程度係受到市場所限制嘅」俾人廣泛噉譽為現代生產理論嘅核心同埋經濟組織嘅根本原則^[141]；亞當史密夫亦都提倡，經濟研究應該假設人類主要係想利己嘅－亞當史密夫話，喺有競爭嘅環境之下，持有資源嘅人絕大多數都會嘗試以對自己最有利嘅方式嚟運用啲資源，而人類偏好利己嘅傾向查實唔係一件壞事，例如佢講嘅呢段嘢就成為咗經濟學上嘅名言^[142]^[143]：

「

原版英文："He generally, indeed, neither intends to promote the public interest, nor knows how much he is promoting it. By preferring the support of domestic to that of foreign industry, he intends only his own security; and by directing that industry in such a manner as its produce may be of the greatest value, he intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. Nor is it always the worse for the society that it was no part of it. By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it."

粵文翻譯：佢（指一個典型嘅人）普遍唔打算造福社會，亦都唔知佢做嘅嘢有幾能夠造福社會。佢透過撐國內工業多過撐國外嘅，目的只係確保自己嘅安全；而佢透過令工業產出價值最大化，目的只係要自己嘅利益，同好多情況一樣，佢（喺呢個情況下）俾一隻無形嘅手引導，令佢達到一個佢冇打算達到嘅結果。呢一點亦都唔係一定係一件壞事。透過追求自己嘅利益，佢好多時都會唔覺意之中更加有效噉造福社會。

」

亞當史密夫「人類主要係想利己嘅」呢個命題係現代經濟學研究嘅基本假設^[144]^[145]。

另一方面，英格蘭學者馬爾薩斯（Thomas Robert Malthus）又提出咗報酬遞減（diminishing return）嘅概念：佢指出隨住生產因素嘅數量增加，邊際產量（marginal productivity；指產量上升幾多）會下降，例如如果員工數量一路係噉升，但其他生產因素（好似機械嘅數量）數量不變，產量嘅提升率會係噉跌，令到請新員工愈嚟愈唔著數；馬爾薩斯運用呢個概念解釋，點解有陣時個經濟體當中會有人工持續處於低水平嘅情況－佢指出喺好多經濟體當中，都有工人數量上升，但其他生產因素冇變嘅情況，而呢個情況會搞到啲老細愈嚟愈冇誘因請工人，自然搞到老細愈嚟愈冇誘因俾高人工^[146]。雖然馬爾薩斯嘅分析受到某啲經濟學家批評^[147]，但報酬遞減嘅概念對今日嘅經濟學嚟講都仲係相當有用^[148]。

比較優勢（comparative advantage）嘅概念都係喺古典經濟學時期出現嘅：喺 1817 年，英國經濟學家大衛·李嘉圖（David Ricardo）專注於分析收入點樣喺土地擁有者、工人、同埋資本家之間分佈。李嘉圖認為，喺土地量不變嘅情況下，隨住人口同資本係噉增加，租金會上漲，搞到人工同利潤停滯唔升，同馬爾薩斯一樣運用咗報酬遞減嘅概念。更加重要嘅係，李嘉圖用數學方式證明咗比較優勢嘅原則^[149]，跟手仲用埋呢條原則嚟解釋點解人同人以至國家同國家之間有誘因進行貿易，而唔係齋靠自己生產嗮所有需要嘅嘢^[150]。

古典經濟學亦都將學界嘅注意力帶到去價值理論（value theory）身上。價值理論係指分析人類個體重視啲乜同埋點解重視呢啲嘢，當中亞當史密夫就寫話「每一樣嘢嘅真正價錢... 反映咗要攞到佢有幾麻煩」（real price of every thing ... is the toil and trouble of acquiring it），所以一樣貨品服務嘅價值唔淨只反映咗要生產呢樣嘢要用嘅錢，仲反映咗生產呢樣嘢所造成嘅其他麻煩－呢點令到經濟學分析生產過程嗰時，唔淨只要考慮生產過程所涉及嘅資本。呢個諗頭喺當時相當有革命性^[151]。

馬克思經濟學

馬克思主義（Marxism）係建基於古典經濟學理論上嘅一套經濟理論。喺 1867 年 10 月 1 號，德國猶太人政經學者馬克思（Karl Marx）出版咗佢嗰三本名著《資本論》（Das Kapital）嘅第一本。喺呢本書入面，馬克思對勞工同資本家對勞工嘅剝削做咗詳細嘅分析。佢認為一件貨品服務嘅價值係由生產佢嘅勞工所決定嘅，而一個勞工所生產嘅價值當中淨係得一部份係去咗個勞工度嘅，其他嗰啲會走咗去資本家嗰度，而呢個過程當中有機會出現對勞工嘅剶削，令到資本家攞走嗰部份唔合理咁大。馬克思嘅思想好有影響力，例如共產主義（communism）就好大程度上建基於馬克思主義之上嘅^[152]^[153]。

馬克思主義嘅其中一個重要主題係「衝突」。馬克思主義假設咗生產嘅過程會影響各種嘅社會現象，包括咗政治組織、法律、文化以至意識形態等，而生產方式係受制於科技等嘅因素嘅。馬克思佢仲主張，隨住生產方式改良，現有嘅生產方式組織往往會變得不合時宜，並且阻礙社會嘅進步，而呢個現象會引致某啲社會衝突，包括咗階級鬥爭（class struggle）：例如如果科技上嘅進步令到機械生產變得更加有效率，有可能會令到啲老細有誘因炒工人魷魚，工人自然就會唔高興，搞到「資本家」同「工人」呢兩個階級之間起衝突，而衝突就會引致革命^[154]。馬克思主義當中嘅階級鬥爭係一個好有影響力嘅概念，就算係今日嘅經濟學研究都仲會用呢個概念嚟對社會現象作出批判^[155]。

新古典經濟學

廿世紀初係經濟學上嘅一個轉捩點－經濟學喺呢個時期由淨係研究財富，變成一個更廣泛嘅領域。早期嘅經濟學都集中於研究財富以及財富嘅生產同分配，例如頭先提過嘅亞當史密夫一味淨係講財富上嘅決策同分析，但喺廿世紀初，經濟學界發現經濟學上嘅原則同理論有好多都可以用落去教育、軍事、同醫療等嘅領域嗰度，於是經濟學家開始覺得舊有對於「經濟學」嘅定義經已唔再夠用^[18]。1932 年，英國經濟學家里奧勞·羅賓斯（Lionel Robbins）出版咗佢篇文章《An Essay on the Nature and Significance of Economic Science》（粵文：論經濟科學嘅本質同重要性）討論到呢個問題，提倡經濟學嘅定位唔應該淨係局限於「對財富嘅研究」，而係應該被視為一個研究人類行為某一部份嘅領域，佢提出稀缺性嘅概念，指出稀缺性迫使人類對點樣運用有限資源做出決擇－令到學界重新將經濟學定義為「研究人類點樣分配有限資源嘅科學領域」，並且正式將用經濟學原理分析財富以外嘅現象嘅研究納入為經濟學嘅一部份。羅賓斯嘅貢獻根本噉改革咗經濟學呢個領域^[156]^[157]^[158]。

新古典經濟學（neoclasical economics）呢套經濟理論係喺 1870 至 1910 年興起嘅^[159]。新古典經濟學研究個體人類、家庭、同組織嘅行為同埋佢哋點樣運用手頭上嘅稀缺資源達到佢哋嘅目的。呢套經濟理論假設一個人大致上係理性嘅，喺任何時間點都喺度嘗試運用手上嘅有限資源嚟達到某啲目的，而且喺決策方面有某啲偏好。一個手上控制住某啲資源嘅個體嘗試喺資訊、時間、同自身認知能力嘅局限之下盡量擴大自己嘅利益，一個社會由多個經濟個體組成，每個個體都會按照自己嘅資源同目的嚟做決策，唔同個體嘅決策會互動，造成各種社會層面嘅經濟現象^[160]，例如係供應同需求噉，當一件貨品服務推出嗰陣，每個個體消費者都要按照件貨品服務嘅價格以及自己手頭上有幾多資源決定要買幾多－而成個市場裏面嘅消費者決定買嘅量就形成咗件貨品服務嘅需求。新古典經濟學理論跟手仲用呢兩個概念嚟解釋市場均衡嘅情況。另一方面，新古典理論放棄咗古典經濟學當中嘅某啲諗法，用新嘅理論分析需求方嘅價值同供應方嘅價值。呢啲做法喺當時係好有革命性，令到經濟學研究變得更加似現代所見到嘅樣^[161]^[162]。

凱恩斯經濟學

凱恩斯經濟學（Keynesian economics）係由英格蘭經濟學家凱恩斯（John Maynard Keynes）所創嘅一套經濟理論。喺 1936 年，凱恩斯出版咗佢本名著《The General Theory of Employment, Interest and Money》（粵文：就業、利息同貨幣通論），呢本書集中討論喺價格相對唔變嘅情況下國家收入由乜嘢因素話事，仲嘗試解釋點解喺某啲情況下，高失業率嘅問題會因為「有效需求」低得滯而唔會自行改善。凱恩斯嘅諗法對經濟分析方法有相當大嘅衝擊，而且仲令到宏觀經濟學成為獨立於微觀經濟學嘅一門經濟學子領域^[163]^[164]。除此之外，凱恩斯經濟學仲有後續嘅學派^[165]。

芝加哥學派

芝加哥經濟學派（The Chicago School of economics）係廿世紀早期興起嘅一套經濟學理論，主張自由市場同埋貨幣主義（monetarism）。根據呢個學派嘅經濟學者（例如係佛利民；Milton Friedman），只要錢供應唔好突然擴充或者收縮，市場經濟體本質上係穩定嘅，而呢個學派嘅分析師普遍認同，錢供應上嘅問題係大蕭條嘅主要成因^[166]。

佛利民等嘅學者攞咗好多古典經濟學上嘅原則，並且將呢啲原則現代化。舉個簡單嘅例子說明，佛利民 1970 年嗰時喺紐約時報寫咗篇專欄，當中指出公司嘅社會責任係「運用手頭上嘅資源，喺唔呃人嘅情況同埋公開嘅競爭之下，將自己嘅利潤最大化」－用咗企業社會責任（corporate social responsibility）呢個現代嘅概念，但又好似古典經濟學噉，主張個人嘅利己行為對社會整體嘅福祉有利^[167]。

行為經濟學

行為經濟學（behavioral economics）係廿世紀中興起嘅一套經濟學理論，專門研究一啲非理性嘅因素－例如係心理、認知、情緒同文化等－會點樣影響個體以至群體嘅經濟決策：直至廿世紀初都霸佔住經濟學研究嘅新古典經濟學理論假設咗，人類係大致理性（rational）嘅，對於每個決定會對自己帶嚟幾多得失曉做出大致上完美嘅運算，並且淨係一味按照運算嘅結果做決策，但打後廿世紀中嘅研究話咁快就令到學界質疑呢個諗法－佢哋發現咗，人類好多時都唔能夠對經濟價值做正確判斷，而且喺做經濟決策嗰時會受好多非理性嘅因素左右^[169]^[170]。

面對呢個問題，行為經濟學理論家提出咗有限理性（bounded rationality）嘅概念：根據有限理性，人類可以係理性嘅決策者，但佢哋喺運算能力、資訊、時間、同記憶等嘅因素上受到限制，所以好多時都唔能夠做完全理性嘅決策。例如假想有個人，佢想用手頭上嘅錢最大噉滿足自己嘅慾望，喺完美理性嘅情況下，佢會集中淨係諗點樣用手上資源達到「最大噉滿足自己慾望」，忽略嗮第啲嘢，而且完美清楚知道每個可能選項有幾能夠滿足自己慾望；而喺有限理性嘅情況下，經濟學家就會考慮埋有啲乜嘢因素可能搞到佢做唔到完美理性嘅決策－比如話，可能嘅使錢方案多得滯，搞到個人唔可以冚唪唥考慮嗮（運算能力不足），又或者嗰個人未必知道嗮所有可能嘅選擇（資訊不足），又或者佢唔夠時間做咁大量嘅運算（時間不足）等等－喺古典經濟理論模型之上加入對「人類所面對嘅限制」嘅考量，務求創造出更加準確描述人類決策嘅理論模型^[171]^[172]。

到咗廿一世紀初為止，行為經濟學經已成為咗一個舉足輕重嘅領域，運用認知科學（cognitive science；研究心靈嘅科學理論，對人類認知能力受嘅限制等課題有相當著墨）等領域嘅知識結合傳統經濟理論，創造咗自己專屬嘅一套理論框架。而且仲有多位嚟自心理學（認知科學嘅其中一個領域）等背景嘅學者憑佢哋嘅研究奪得諾貝爾經濟學獎嘅殊榮^[173]^[174]。

批評

假設真實度

有唔少學者都批評經濟學嘅假設，話呢啲假設好多時都係簡化得滯嘅。喺任何嘅理論分析（包括經濟學）當中，研究者梗要做一啲簡化，集中思考對研究現象嚟講最緊要嗰啲變數，但亦都因為噉有陣時要焗住做某啲假設，令到個理論模型嘅真實性受質疑。學者需要爭論到底一個理論模型有幾能夠代表到現實，同個模型「代表到現實」嘅程度係咪夠高，可唔可以攞嚟預測現實世界發生嘅嘢^[175]^[176]。

舉個例說明：喺做博弈論嘅分析嗰時，個研究者會假設所有博弈參與者都想將自己贏嘅機會率最大化，佢跟手會用個理論模型分析現實世界嘅現象；問題事實係，喺現實當中，的確會有一部份嘅人會因為某啲原因而唔想喺場博弈當中贏，所以佢個理論模型只不過係對現實嘅大致（approximate）描述，唔完全代表到現實；只要大部份人都係想贏嘅，噉用佢呢個理論做分析會大致上正確，但問題係，到底想贏嘅人佔咗幾多百分比？原則上，呢個百分比愈高，個理論模型就愈會做到準確嘅預測。而到底呢個百分比要幾高，個模型先至算「啱用」^[175]？

好多批評經濟學研究方法嘅學者都係講緊呢樣嘢：佢哋質疑到底經濟學理論做嗰啲假設係咪有足夠嘅真實度，確保到啲理論嘅實用性。除咗頭先提到嘅博弈論例子，「人類大致上係理性嘅」同埋「人類具有足夠嘅資訊做正確決策」等都係啲經濟理論模型常用嘅假設，佢哋嘅可用性同樣受到廣泛爭議（睇埋行為經濟學）^[177]。

預測能力

有唔少主流經濟學家（例如係凱恩斯）都指出，經濟學理論好多時都流於概念性嘅分析，做唔到量化嘅預測。喺科學上，一個理想嘅理論模型應該要準確噉預測現實世界嘅現象，例如牛頓力學嗰柞定律就可以準確噉預測速度同力等變數之間嘅關係，令到人可以知道咗（例如）一件物體所受嘅力同佢嘅質量之後，近乎完美噉預測件物體速度嘅變化^[178]。

經濟學都嘗試做類似嘅嘢，用某啲數字預測第啲數字，例如靠一件貨品服務嘅價格預測生產者最後會得到嘅盈利，但經濟體係一個複雜嘅系統，而且經濟學研究嘅變數好多時都冇辦法直接量度（例如消費者「買某件貨品嘅意願」頂櫳只可以靠問卷調查嚟量度）。結果係，呢類預測嘅準確性通常都冇物理學等「硬」科學做嘅預測咁準。好多時，經濟學只係做到預測邊個邊個變數同邊個邊個變數成正比定反比－即係話呢啲理論嘅預測能力冇硬科學上嘅理論咁勁^[179]^[180]。

喺實證經濟學研究上，一份研究嘅再現性（replicability）係一個受關注嘅課題。再現性係指一份科學研究俾嘅結果應該係客觀嘅，所以如果第啲人用同樣嘅方法做嗰份研究嘅話，理應會攞到相同嘅結果^[181]^[182]，但係經濟學嘅研究時不時俾人發現係再現唔到嘅，而且經濟學期刊成日俾人插，話佢哋唔肯刊登一啲嘗試做再現嘅研究（因為嫌呢啲研究冇乜話題性）。噉亦都一定程度上因為經濟學（同第啲社會科學）一樣，都係研究緊複雜系統嘅，呢啲系統內有好多因素，而且唔同因素之間仲會互相影響，搞到好多時好難控制嗮所有嘅因素同用完全同樣嘅方法重做一份研究^[183]^[184]。

註釋

↑ 順帶一提，如果家陣有兩件貨品，消費者對兩者嘅需求會一齊升或者一齊跌，噉呢兩件貨品就算係所謂嘅互補品（complementary goods）；例子有牙刷同牙膏。
↑ 兩件貨品 $A$ 同 $B$ 係互補品定替代品可以用以下呢條式衡量：

$CG={\frac {\Pr(A\cap B)}{\Pr(A\cup B)}}\,$
$\Pr(A\cap B)$ 係「消費者買嗮兩件貨」嘅機率，而 $\Pr(A\cup B)$ 係「消費者買嗮兩件或者淨係買其中一件」嘅機率，如果 $A$ 同 $B$ 係互補品， $CG$ 數值會大，而如果 $A$ 同 $B$ 係替代品， $CG$ 數值會細。

睇埋

文獻

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拎

Economics．不列顛百科全書對經濟學嘅解釋。
Economic journals on the web.
Intute: Economics: Internet directory of UK universities.
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Economics Departments, Institutes and Research Centers in the World.
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[82] 順帶一提，如果家陣有兩件貨品，消費者對兩者嘅需求會一齊升或者一齊跌，噉呢兩件貨品就算係所謂嘅互補品（complementary goods）；例子有牙刷同牙膏。

[83] 兩件貨品 $A$ 同 $B$ 係互補品定替代品可以用以下呢條式衡量：

$CG={\frac {\Pr(A\cap B)}{\Pr(A\cup B)}}\,$
$\Pr(A\cap B)$ 係「消費者買嗮兩件貨」嘅機率，而 $\Pr(A\cup B)$ 係「消費者買嗮兩件或者淨係買其中一件」嘅機率，如果 $A$ 同 $B$ 係互補品， $CG$ 數值會大，而如果 $A$ 同 $B$ 係替代品， $CG$ 數值會細。

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