Yet Aristotle and the schoolmen after him adopt Plato's doctrine that we know by similitude, or by ideas in the sense of images, or representations, interposed between the mind and the object, or thing existing _a parte rei_. They suppose these images, or intelligible species, form a sort of intermediary world, called the _mundus logicus_, distinguished from the _mundus physicus_, or real world, which they are not, but which they image or represent to the understanding. Hence the categories or praedicaments are neither forms of the subject nor forms of the object, but the forms or laws of logic or this intermediary world. Hence has arisen the question whether our knowledge has any objective validity, that is, whether there is any objective reality that responds to the idea. Perhaps it is in this doctrine, misunderstood, that we are to seek the origin of scepticism, which always originates in the speculations of philosophers, never in the plain sense of the people, who never want, when they know, any proof that they know. This Platonic and peripatetic doctrine, that ideas are not the reality, but, as Locke says, that "with which the understanding is immediately conversant," has been vigorously assailed by the Scottish school, which denies intermediary ideas, and maintains that we perceive directly and immediately things themselves. Still the old doctrine obtains to a very considerable extent, and respectable schools teach that ideas, if not precisely images, are nevertheless representative, and that the idea is the first object of mental apprehension. Balmes never treats ideas as the object existing _in re_, but as its representation to the mind. Hence the importance attached to the question of certainty, or the objective validity of our knowledge, around which Balmes says turn all the questions of philosophy; that is, the great labor of philosophers is to prove that in knowing we know something, or that to know is to know. This is really the _pons asinorum_ of modern philosophy as it was of ancient philosophy: How know I that knowing is knowing, or that in knowing I know? The question as asked is unanswerable and absurd, for I have only to know with which to prove that I know, and he who knows knows that he knows. I know that I know says no more than I know.