MIND AND SOUL Practical reasoning is a diYcult topic, and its logic has to this day not been fully worked out One way in which it diVers from theoretical reasoning is that it is, in the lawyer’s jargon, defeasible What that means is this In theoretical deductive reasoning, if a conclusion follows from a given set of premisses it follows also from any larger set containing those premisses: the argument cannot be defeated by the addition of an extra premiss But with practical reasoning it is diVerent A pattern of reasoning that would justify a certain course of action on the basis of certain wants and beliefs may well cease to justify it if further wants and beliefs are brought into consideration Aquinas recognized the defeasibility of practical reasoning, and indeed he saw it as the underlying ground of the freedom of the will In human beings, unlike animals, he says, Because a particular practical evaluation is not a matter of inborn instinct, but a result of weighing reasons, a human being acts upon free judgement, and is capable of going various ways In contingent matters reason can go either way and what to in particular situations is a contingent matter So in such cases the judgement of reason is open to alternatives and is not determined to any one course Hence, humans enjoy free decision, from the very fact of being rational (ST 1a 83 1c) When we look at a piece of practical reasoning—reasoning about what to do—we Wnd, where the analogy of theoretical reasoning would lead us to expect necessitation, merely contingent and defeasible connections between one step and another Aquinas believed that this contingency was the fundamental ground of human freedom Aquinas does not generally employ a Latin expression corresponding to our ‘freedom of the will’: he talks instead of the will (voluntas) and of ‘free choice’ (liberum arbitrium) Choice is an expression of both the intellect and the will: it is an exercise of the intellect because it is the fruit of reasoning; it is an exercise of the will because it is a form of appetition Following Aristotle, Aquinas tells us that it is both appetitive intelligence, and ratiocinative appetite (ST 1a 83c) Intellect and will are the two great powers of the rational soul, the soul that is peculiar to human beings Besides being the soul that only human beings have, it is the only soul that human beings have Against those contemporaries who thought that humans had also animal and vegetable 240