On the So-called Logic of Practical Inference

Summary

Different questions generate different forms of practical reasoning. A contextually unrestricted ‘What shall I do?’ is too open to focus reflection. More determinately, an agent may ask, ‘Shall I do X, or Y?’ To answer that, he may need to weigh things up—as fits the derivation of ‘deliberation’ from libra (Latin for ‘scales’). Ubiquitous and indispensable though this is, I mention it only to salute it in passing.¹ Or he may ask how to achieve a proposed end: if his end is to do X, he may ask ‘How shall I do X?’ Or he may ask how to apply a universal rule or particular maxim.² Aristotle supplies examples in De Motu Animalium (7.701a7 ff.), whose wording I freely adapt to my own purposes:

• A1 reasons to a necessary means to achieving an end:

  • I will make a cloak.

  • To make a cloak I must do A.

  • So, I will do A.

• A2 reasons to a sufficient means to achieving an end:

  • I will make something good.

  • A house is something good.

  • So, I will make a house.

• B1 applies a universal rule:

  • Every man must walk.

  • I am man.

  • So, I must walk.

• B2 applies a conditional that speaks of a particular agent at a certain time:

  • I will now make a cloak if I need one.

  • I need a cloak.

  • So, I will now make a cloak.

A paraphrase of ‘Every man must walk’ may bring out the affinity of B1 with B2: ‘For all x, if x is a man, x must walk.’ Both, therefore, involve reasoning from a hypothetical intention or requirement to a categorical one.