For example, the following is a very general truth: the day succeeds the night; but is it a universal and necessary truth? Does it extend to all lands? Yes, to all known lands. But does it extend to all possible lands? No; for it is possible to conceive of lands plunged in eternal night, another system of the world being given. The laws of the material world are what they are; they are not necessary. Their Author might have chosen others. With another system of the world one conceives other physics, but we cannot conceive other mathematics and other morals. Thus it is possible to conceive that day and night may not be in the same relation to each as that in which we see them; therefore the truth that day succeeds night is a very general truth, perhaps even a universal truth, but by no means a necessary truth. Montesquieu has said that liberty is not a fruit of warm climates. I acknowledge, if it is desired, that heat enervates the spirit, and that warm countries maintain free governments with difficulty; but it does not follow that there may be no possible exception to this principle: moreover, there have been exceptions; hence it is not an absolutely universal principle, much less is it a necessary principle. Could you say as much of the principle of cause? Could you in any way conceive, in any time and in any place, a phenomenon which begins to appear without a cause, physical or moral? And were it possible to reduce universal and necessary principles to general principles, in order to employ and apply these principles thus abased, and to found upon them any reasoning whatever, it would be necessary to admit what is called in logic the principle of contradiction, viz., that a thing cannot at the same time be and not be, in order to maintain the integrity of each part of the reasoning; as well as the principle of sufficient reason, which alone establishes their connection and the legitimacy of the conclusion. Now, these two principles, without which there is no reasoning, are themselves universal and necessary principles; so that the circle is manifest. Even were we to destroy in thought all existences, save that of a single mind, we should be compelled to place in that mind, in order that it might exercise itself at all--and the mind is such only on the condition that it thinks--several necessary principles; it would be beyond the power of thought to conceive it deprived of the principle of contradiction and the principle of sufficient reason.