The greatest obstacle to the advancement of human knowledge, lies not so much in the things themselves, as in our manner of considering them. However complicated the body of man may be, his ideas are more so. It is less difficult to understand Nature as she is, than comprehend her as she is represented. She has only a veil, but we give her a mask, and conceal her with prejudices; and we suppose she acts and operates as we act and think; but her actions however are clear, and our thoughts are obscure; her designs and operations are always uniform and certain, which we seem to confound with the variable illusions of our own imaginations. I speak not merely of arbitrary systems and imaginary hypotheses, but of the methods by which we generally study Nature. Even experiment, although the most certain method, has been productive of more error than truth; as the smallest deviation leads to barren wilds, or exhibits a glimpse of obscure objects; to which affinities and properties are ascribed, and those steps being followed by the whole world, the consequences derived from them are admitted as fixed principles. Of this I might give a proof by exposing what are called principles in all the sciences, both abstract and real. In the former the general basis of principle is abstraction, or one or more suppositions; in the latter, principles are nothing more than consequences, whether true or false, of the methods which we have adopted. Let us take anatomy for an example: must not the first man who surmounted natural repugnance, and ventured to open a human body, suppose that by dissecting and examining all its parts, he should obtain a knowledge of its structure, mechanism and functions? but finding the subject more complicated than he had imagined, he was obliged to renounce those pretensions, and to adopt a method, not by which he might know and judge, but by which he might view the parts in a certain order. This method, however, was not to be acquired by one man; it was to occupy the attention of ages, and even of our ablest anatomists to the present day, and even when acquired it is not science, but the road which leads to it; and which might have done so, if instead of keeping within the narrow and beaten track, anatomists had extended the path, by comparing the human body with that of other animals; for does not the foundation of all science consist in a comparison of similar and different objects, of their analogous and opposite properties, and of all their relative qualities? And hence it is, that although human bodies have been dissected for three thousand years, anatomy still remains nothing more than a nomenclature, and hardly any advances have been made towards the real object, the knowledge of the animal economy; in which Nature certainly appears very mysterious, not only because the subject is complicated, but because, having neglected those modes of comparison, which alone could have