XVIII. It is certain there are some truths, that maugre the clearness and confidence with which the understanding assents to them, are not exempt from being exposed to difficult objections; or to speak more properly, there is no truth whatever, be it ever so firmly established, against which, some sophistical objection may not be raised. It would therefore not be right upon all occasions, to give up a maxim, whose truth we clearly perceive and are convinced of, only because we can’t reply to every argument that may be urged against it. There are truths of such a nature, that although they would strike, and be clear to any man of ordinary understanding, still, the giving an answer to every objection that could be raised against them, might possibly be found a difficult task to a person of very subtil ingenuity. And notwithstanding we should not be able to hit upon an argument, that would confront or silence those which are used by the sceptics, to persuade us to doubt whether we are sleeping or waking, we should not abandon ourselves to such a doubt, but support ourselves in a firm confidence, that we are not mistaken with respect to the state we are in. And in truth, the arguments used to make us doubt of it, are not of such a nature, as that a clear, solid, and unembarrassed answer may not be given to them. XIX. And in order to do this, I shall begin with supposing, that evidence in this case may be of two sorts, mediate, and immediate. An evident proposition, supported by immediate evidence, is, when without the assistance of any kind of proof, it presents itself so clearly to our understandings, that the mind is constrained by invincible necessity to assent to it. An evident proposition supported by mediate evidence, is, when of itself, it is not represented to our understandings with all this clearness, but is necessarily inferred from some other self-evident proposition. XX. I will suppose, secondly, that immediate evidence should be divided, into metaphysical and experimental. The first is governed by universal principles, which of themselves persuade or convince the understanding; such as these, the whole is greater than a part; two contradictory propositions can’t be both true at one and the same time. Experimental evidence is derived from certain singular truths, which strike every individual with infallible conviction; such as these, that I at present have such or such a desire, or that I think of such or such a thing, that I suffer some pain, or that I am affected with some particular sensation; for example, joy, sadness, or anger.