1. The Essence or Quiddity (Form)--[Greek: to\ ti/ ê)=n ei)=nai]. 2. The necessitating conditions (Matter)--[Greek: to/ ti/nôn o)/ntôn a)na/gkê tou=t' ei)=nai]. 3. The proximate mover or stimulator of change (Efficient)--[Greek: ê( ti/ prô=ton e)ki/nêse]. 4. That for the sake of which (Final Cause or End)--[Greek: to\ ti/nos e(/neka]. All these four Causes (Formal, Material, Efficient, Final) appear as middle terms in demonstrating. We can proceed through the medium either of Form, or of Matter, or of Efficient, or of End. The first of the four has already been exemplified--the demonstration by Form. The second appears in demonstrating that the angle in a semi-circle is always a right angle; where the middle term (or matter of the syllogism, ([Greek: to\ e)x ou(=]) is, that such angle is always the half of two right angles.[26] The Efficient is the middle term, when to the question, Why did the Persians invade Athens? it is answered that the Athenians had previously invaded Persia along with the Eretrians. (All are disposed to attack those who have attacked them first; the Athenians attacked the Persians first; _ergo_, the Persians were disposed to attack the Athenians.) Lastly, the Final Cause serves as middle term, when to the question, Why does a man walk after dinner? the response is, For the purpose of keeping up his health. In another way, the middle term here is digestion: walking after dinner promotes digestion; digestion is the efficient cause of health.[27] [Footnote 26: Analyt. Post. II. xi. p. 94, a. 21-36. Themistius, p. 83: [Greek: ma/lista me\n ga\r e)pi\ pa/sês a)podei/xeôs o( me/sos e)/stin oi(=on ê( u(/lê tô=| sullogismô=|; ou(/tos ga\r o( poiô=n ta\s du/o prota/seis, e)ph' ai(=s to\ sumpe/rasma.]] [Footnote 27: Analyt. Post. II. xi. p. 94, a. 36-b. 21.] The Final Cause or End is prior in the order of nature, but posterior to the terms of the conclusion in the order of time or generation; while the Efficient is prior in the order of time or generation. The Formal and Material are simultaneous with the effect, neither prior nor posterior.[28] Sometimes the same fact may proceed both from a Final cause, and from a cause of Material Necessity; thus the light passes through our lantern for the purpose of guiding us in the dark, but also by reason that the particles of light are smaller than the pores in the glass. Nature produces effects of finality, or with a view to some given end; and also effects by necessity, the necessity being either inherent in the substance itself, or imposed by extraneous force. Thus a stone _falls_ to the ground by necessity of the first kind, but _ascends_ by necessity of the second kind. Among products of human intelligence some spring wholly from design without necessity; but others arise by accident or chance and have no final cause.[29]