it is necessary to add, that this question must be answered in the negative. Whether or not we can imagine, or mentally represent, a thing as real, depends upon the question whether our past experience has furnished us the data for such a representation; and our experience is constantly furnishing us new data.
That the impossibility or difficulty of imagining a thing (which, however, must be carefully distinguished from the absolute impossibility of forming certain concepts, of which I shall presently speak) is no evidence for or against its reality, is a truth of the greatest moment to the student of natural science. Liebig expressed it long ago (Ann. Pharm., x., 179), in the words: "The secret of all those who make discoveries is, that they regard nothing as impossible."
I come now to the conditions of conceivability, strictly and properly so called. These conditions are readily deduced from the incidents in the act of conception to which I have referred. These incidents are: The reduction of the elements of a representation to consistent unity by bringing them into relation, and the establishment of relations between the unit thus evolved and the previous concepts of the mind. A concept can, therefore, be formed, if a, its elements, can be united in thought by the establishment of relations between them by which they are reduced to a unit—in other words, if the constituent attributes are consistent with each other—and if b, the resulting concept, can be brought into relation, so as to be consistent with the previously-formed concepts of the mind.
Consistency of the constituent attributes with each other, therefore, is the first, and consistency of the concept with other concepts the second, condition of its successful formation. The first of these is what is known in logic as the law of non-contradiction, or the law of consistency, and is the fundamental condition of all thought. It requires that what is expressly or by implication asserted in the subject shall not expressly or by implication be denied in the predicate of any proposition into which the concept may be resolved, or, in plain words, that what is asserted in one form of words shall not be denied in another.
Now, it is evident that, whenever the formation of a concept involves a violation of the first condition, we have before us a case of absolute inconceivability, and therefore of impossibility. For this condition, as I have said, is the first constitutive law of all intelligence, without which the whole system of relations, in which both subjective and objective realities have their only warrant and support, instantly collapses into the nothingness in which alone all things are identical, and disappears in the night of absolute confusion. No one, not even John Stuart Mill, ever seriously doubted the absolute impossibility of the conception or existence of a round square, or of a straight line which is not the shortest distance between two points. Whenever such a doubt has been expressed, it has arisen from a mental con-
