‘I would willingly suggest some subject on which I might have the favour of your thoughts when you have leisure and are disposed for such correspondence. What occurs to my thoughts just now is a metaphysical axiom very generally adopted, and, I think, occasionally adopted by you in your _Review of the Principal Questions in Morals_—That what we distinctly conceive is possible. From this axiom D. Hume infers that it is possible that an universe may start into existence without a cause, and other like extravagancies. The use he makes of it led me to consider it a good many years ago, and I have a strong suspicion that there is some fallacy in it, which has imposed on men’s understandings, owing to the ambiguity of the word _conceive_. ‘As to the history of this axiom, I suspect it to have taken its rise from what Descartes laid down as the criterion of truth, which he maintained to be clear and distinct conception. _Quidquid clare et distincte concipio esse verum, id est verum._ Cudworth seems to have followed him in this, making the criterion of verity to be clear, self-consistent intelligibility. Those who came after, judging it difficult to maintain that everything is _true_ which is clearly conceivable or intelligible, have maintained that everything that can be clearly conceived is at least _possible_. This seemed to be the proper correction of Descartes’ maxim, and it has passed from one hand to another without strict examination. ‘Whatever is true or false, whatever is possible or impossible, may be expressed by a proposition. Now, what do we mean when we say that we conceive a proposition? I think no more is meant, if we speak properly, than that we understand what is meant by that proposition. If this be so, it must surely be granted that we may understand the meaning of a proposition which expresses what is impossible. He who understands the meaning of this proposition, “Two and two make four,” must equally understand the meaning of this other, “Two and two do not make four.” Both are equally understood; that is, the conception of both is equally clear; yet the first is necessarily true, and the second impossible.