The question proposed for consideration is such that, if it is appropriately answered, higher philosophy must as a result acquire a determinate form. If the method for attaining the highest possible degree of certainty in this type of cognition has been established, and if the nature of this kind of conviction has been properly understood, then the following effect will be produced: the endless instability of opinions and scholarly sects will be replaced by an immutable rule which will govern didactic method and unite reflective minds in a single effort. It was in this way that, in natural science, Newton's method transformed the chaos of physical hypotheses into a secure procedure based on experience and geometry. But what method is this treatise itself to adopt, granted that it is a treatise in which metaphysics is to be shown the true degree of certainty to which it may aspire, as well as the path by which the certainty may be attained? If what is presented in this treatise is itself metaphysics, then the judgement of the treatise will be no more certain than has been that science which hopes to benefit from our inquiry by acquiring some permanence and stability; and then all our efforts will have been in vain. I shall, therefore, ensure that my treatise contains nothing but empirical propositions which are certain, and the inferences which are drawn immediately from them. I shall rely neither on the doctrines of the philosophers, the uncertainty of which is the very occasion of this present inquiry, nor on definitions, which so often lead to error. The method I shall employ will be simple and cautious. Some of the things I shall have to say may be found to be lacking in certainty; but such things will only have an elucidatory function and will not be employed for purposes of proof.