MOST persons are familiar with the distinction of _necessary_ and _contingent_ truths. The former kind are Truths which cannot but be true; as that 19 and 11 make 30;--that parallelograms upon the same base and between the same parallels are equal;--that all the angles in the same segment of a circle are equal. The latter are Truths which _it happens_ (_contingit_) are true; but which, for anything which we can see, might have been otherwise; as that a lunar month contains 30 days, or that the stars revolve in circles round the pole. The latter kind of Truths are learnt by experience, and hence we may call them _Truths of Experience_, or, for the sake of convenience, _Experiential_ Truths, in contrast with Necessary Truths. Geometrical propositions are the most manifest examples of Necessary Truths. All persons who have read and understood the elements of geometry, know that the propositions above stated (that parallelograms {26} upon the same base and between the same parallels are equal; that all the angles in the same segment of a circle are equal,) are necessarily true; not only they are true, but they _must be_ true. The meaning of the terms being understood, and the proof being gone through, the truth of the propositions must be assented to. We learn these propositions to be true by demonstrations deduced from definitions and axioms; and when we have thus learnt them, we see that they could not be otherwise. In the same manner, the truths which concern numbers are necessary truths: 19 and 11 not only _do_ make 30, but _must_ make that number, and cannot make anything else. In the same manner, it is a necessary truth that half the sum of two numbers added to half their difference is equal to the greater number. It is easy to find examples of Experiential Truths;-- propositions which we know to be true, but know by experience only. We know, in this way, that salt will dissolve in water; that plants cannot live without light;-- in short, we know in this way all that we do know in chemistry, physiology, and the material sciences in general. I take the _Sciences_ as my examples of human knowledge, rather than the common truths of daily life, or moral or political truths; because, though the latter are more generally interesting, the former are much more definite and certain, and therefore better starting-points for our speculations, as I have already said. And we may take elementary astronomical truths as the most familiar examples of Experiential Truths in the domain of science.