Since reasoning is somewhat variable, and is in a way mobile, according as it proceeds from principles to conclusions, and since deception is possible in this process of reasoning, every act of reasoning must proceed from some knowledge that has a definite uniformity and stability. Such knowledge does not come about by discursive investigation; rather, it is presented all at once to the intellect. Just as reasoning in the speculative area goes deductively from certain self-evident principles...so also must practical reasoning make its deductions from self-evident principles (for example, that evil should not be done, that the precepts of God will have to be obeyed, and others such) (St. Thomas, Commentary on the Sentences, II, 24, 2, 3, quoted in Bourke, The Pocket Aquinas, p. 199).These principles are either empirically arrived at, or else are immediately obvious to anyone once they are presented to him (e.g., that the whole is greater than any of its parts).
If we do not start our reasoning in such a way, then there is no anchor for it at all, and consequently we may stumble into error or deception.
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