Since then sciences are the results of mental processes about one and the same subject-matter, viewed under its various aspects, and are true results, as far as they go, yet at the same time separate and partial, it follows that on the one hand they need external assistance, one by one, by reason of their incompleteness, and on the other that they are able to afford it to each other, by reason, first, of their independence in themselves, and then of their connexion in their subject-matter. Viewed altogether, they approximate to a representation or subjective reflection of the objective truth, as nearly as is possible to the human mind, which advances towards the accurate apprehension of that object, in proportion to the number of sciences which it has mastered; and which, when certain sciences are away, in such a case has but a defective apprehension, in proportion to the value of the sciences which are thus wanting, and the importance of the field on which they are employed.
Let us take, for instance, man himself as our object of contemplation; then at once we shall find we can view him in a variety of relations; and according to those relations are the sciences of which he is the subject-matter, and according to our acquaintance with them is our possession of a true knowledge of him. We may view him in relation to the material elements of his body, or to his mental constitution, or to his household and family, or to the community in which he lives, or to the Being who made him; and in consequence we treat of him respectively as physiologists, or as moral philosophers, or as writers of economics, or of politics, or as theologians. When we think of him in all these relations together, or as the subject at once of all the sciences I have named, then we may be said to reach unto and rest in the idea of man as an object or external fact, similar to that which the eye takes of his outward form. On the other hand, according as we are only physiologists, or only politicians, or only moralists, so is our idea of man more or less unreal; we do not take in the whole of him, and the defect is greater or less, in proportion as the relation is, or is not, important, which is omitted, whether his relation to God, or to his king, or to his children, or to his own component parts. And if there be one relation, about which we know nothing at all except that it exists, then is our knowledge of him, confessedly and to our own consciousness, deficient and partial, and that, I repeat, in proportion to the importance of the relation.