See especially his chapter ii. on the Sensations of Sight, pp. 222, 241—247, in the second edition of this work.
Descartes says, in his 'Principia Philosophiæ,' i 51—'Et quidem substantia quæ nullâ planè re indigeat, unica tantum potest intelligi—nempe Deus. Alias vero omnes, non nisi ope concursûs Dei existere posse perspicimus. Atque ideo nomen substantiæ non convenit Deo et illis univocè, ut dici solet in scholis, hoc est, nulla ejus nominis significatio potest distinctè intelligi, quæ Deo et creaturis sit communis.'
At the same time, we cannot go along with Mr Mill in the following affirmation (p. 201):—
'This natural probability is converted into certainty when we take into consideration that universal law of our experience which is termed the Law of Causation, and which makes us unable to conceive the beginning of anything without an antecedent condition, or cause.' Such 'inability to conceive' appears to us not in correspondence with facts. First, it cannot be properly either affirmed or denied, until agreement is obtained what the word cause means. If three persons, A, B, and C, agree in affirming it—A adopting the meaning of Aristotle, B that of Sir William Hamilton, and C that of Mr Mill—the agreement is purely verbal; or rather, all three concur in having a mental exigency pressing for satisfaction, but differ as to the hypothesis which satisfies it.