fusion as to the import of the terms employed in the propositions, as we shall see presently in the case of Mill.
But it is otherwise with the second condition of conceivability: that the concept, when framed, shall be consistent with other concepts previously formed. For these latter concepts may be spurious or invalid. Inconceivability arising from non-compliance with the second condition is therefore purely relative, depending on the validity of the concepts with which the concept in question appears to be incompatible. For example, until the discovery of the composition of water, of the true theory of combustion, and of the relative affinities of potassium and hydrogen for oxygen, it was impossible to conceive a substance which would ignite on contact with water, it being one of the recognized attributes of water––in other words, a part of the concept water—that it antagonized fire. This previous concept was spurious, and, when it had been destroyed, the inconceivability of a substance like potassium disappeared. Similarly, we are now unable to conceive a warm-blooded animal without a respiratory system, because we conceive the idiothermic condition of an animal organism to depend mainly on the chemical changes taking place within it, chief among which is the oxidation of the blood, which requires some form of contact between the blood and the air, and therefore some form of respiration. If, however, future researches should destroy this latter concept—if it should be shown that the heat of a living body may be produced in sufficient quantity by mechanical agencies, such as friction—a non-respiring warm-blooded animal would at once become conceivable.
Mill not only refuses to recognize the distinction between what may be conceived and what may be represented in imagination, but he also ignores the distinction between the cases of inconceivability from the one or the other of the two causes just mentioned; and he maintains that all conceivability whatever is relative. The examples which he discusses at length are all cases of inconceivability, and not of unimaginability, and I propose to notice the more important of them in passing. The most noteworthy of these examples is the inconceivability of a round square. In order not to do Mill injustice, it will be best to quote his own language ("Examination of the Philosophy of Sir W. Hamilton," vol. i., p. 88, et seq., American edition):
"We cannot conceive a round square," says Mill, "not merely because no such object has ever presented itself in our experience, for that would not be enough. Neither, for any thing we know, are the two ideas in themselves incompatible. To conceive a round square, or to conceive a body all black and yet all white, would only be to conceive two different sensations as produced in us simultaneously by the same object—a conception familiar to our experience—and we should probably be as well able to conceive a round square as a hard square, or a heavy square, if it were not that in our uniform experience, at the instant when a thing begins to be round, it ceases to be square,
