It is an axiom in mental philosophy, that we can think of nothing which we have not perceived. When I say that we can think of nothing, I mean, we can imagine nothing, we can reason of nothing, we can remember nothing, we can foresee nothing. The most astonishing combinations of poetry, the subtlest deductions of logic and mathematics, are no other than combinations which the intellect makes of sensations according to its own laws. A catalogue of all the thoughts of the mind, and of all their possible modifications, is a cyclopedic history of the universe.
But, it will be objected, the inhabitants of the various planets of this and other solar systems; and the existence of a Power bearing the same relation to all that we perceive and are, as what we call a cause does to what we call effect, were never subjects of sensation, and yet the laws of mind almost universally suggest, according to the various disposition of each, a conjecture, a persuasion, or a conviction of their existence. The reply is simple; these thoughts are also to be included in the catalogue of existence; they are modes in which thoughts are combined; the objection only adds force to the conclusion, that beyond the limits of perception and thought nothing can exist.
Thoughts, or ideas, or notions, call them what you will, differ from each other, not in kind, but in force. It has commonly been supposed that those distinct thoughts which affect a number of persons, at regular intervals, during the passage of a multitude of other thoughts, which are called REAL or EXTERNAL OBJECTS, are totally different in kind from those which affect only a few persons, and which recur at irregular intervals, and are usually more obscure and indistinct, such as hallucinations, dreams, and the ideas of madness. No essential distinction between any one of these ideas, or any class of them, is founded on a correct observation of the nature of things, but merely on a consideration of what thoughts are most invariably subservient to the security and happiness of life; and if nothing more were expressed by the distinction, the philosopher might safely accommodate his language to that of the vulgar. But they pretend to assert an essential difference, which has no foundation in truth, and which suggests a narrow and false conception of universal nature, the parent of the most fatal errors in speculation. A specific difference between every thought of the mind, is, indeed, a necessary consequence of that law by which it perceives diversity and number; but a generic and essential difference is wholly arbitrary. The principle of the agreement and similarity of all thoughts, is, that they are all thoughts; the principle of their disagreement consists in the variety and irregularity of the occasions on which they arise in the mind. That in which they agree, to that in which they differ, is as everything to nothing. Important distinctions, of various degrees of force, indeed, are to be established between them, if they were, as they may be, subjects of ethical and economical discussion; but that is a question altogether distinct. By considering all knowledge as bounded by perception, whose operations may be indefinitely combined, we arrive at a conception of Nature inexpressibly more magnificent, simple and true, than accords with the ordinary systems of complicated and partial consideration. Nor does a contemplation of the universe, in this comprehensive and synthetical view, exclude the subtlest analysis of its modifications and parts.
A scale might be formed, graduated according to the degrees of a combined ratio of intensity, duration, connexion, periods of recurrence, and utility, which would be the standard, according to which all ideas might be measured, and an uninterrupted chain of nicely shadowed distinctions would be observed, from the faintest impression on the senses, to the most distinct combination of those impressions; from the simplest of those combinations, to that mass of knowledge which, including our own nature, constitutes what we call the universe.