XXI. That every individual feels this experimental evidence in some particular instances, no one can doubt, and even if any person, by giving to his scepticism all imaginable extension, should be desirous of doubting of every thing, he could never stifle the experimental evidence that he doubted. And here I can’t help remarking, that experimental evidence is a thing of such moment to the Carthesians, that all their metaphysical testimonies depend on, or are derived from it; because, from that first maxim or proposition, I think, is inferred immediately, the proof of their own existence; and mediately, all the other demonstrable truths, are established upon experimental evidence only. XXII. It is also certain, that from the truths which are established upon experimental evidence, you can deduce no demonstrative reasoning; or at least of that sort, which the logicians call _à priori_. The reason is, because they are evident of themselves, or else are supported by immediate evidence, and are not dependant upon any other thing from whence they are inferred. Therefore, although for example, I at present am convinced that I am desirous of eating such or such a thing, I cannot persuade any other person that I fancy it, by any immediate demonstration; because this is evident to me, not from any principle that is notorious to all mankind, and from whence may be inferred the existence of such a desire; but because the appetite itself, is intimately connected with my spirit, and I feel it so forcibly, that I cannot doubt of its existence: the same thing happens in the cases of those truths, which are established upon immediate metaphysical evidence. If I should be asked, how I know that the whole is greater than one of its parts, I should answer, that I do not know it from any principle from whence it may be inferred, but because the truth, _that the whole is larger than a part_, is so clearly evident to my mind, that it is as incompatible with doubt, as the light of the sun with midnight darkness. If any one should deny to me, the truth of the maxim, that two contradictory propositions cannot be both true at the same time, it would be impossible for me to prove the truth of the maxim, either by arguments deduced _à priori_, or _à posteriori_. The reason is plain, because if I was to argue the point, the most I could do, would be insisting on the manifest contradiction, and asserting the impossibility of the thing, which is the last resource of logical dialect. But observe, when we are got thus far, my adversary, to act consistently with his first caprice, thinks fit to change his ground, and to admit both extremes of the contradiction, and to insist they are both true. Now how, in this case, am I to prove that they cannot be so? Why I can do it by no other means, than by having recourse to the axiom, that it is impossible two contradictory propositions can be both true at the same time. But