Some, indeed, distinguish between "primary" and "secondary" qualities, and hold that the former, such as extension, figure, motion, and solidity, have some existence outside of the mind in an unthinking substance which they call "matter." But extension, figure, and motion are only ideas existing in the mind, and neither these ideas nor their archetypes can exist in an unperceiving substance. The very notion of what is called "matter" involves a contradiction within it. Not only primary and secondary qualities alike, but also "great" and "small," "swift" and "slow," "extension," "number," and even "unity" itself, being all of them purely relative, exist only in the mind. The conception of "material substance" has no meaning but that of "being" in general. Even if we were to give to the materialists their "external bodies," they are by their own confession no nearer to knowledge how our ideas are produced, since they own themselves unable to comprehend in what manner body can act upon spirit, or how it is possible that it should imprint any idea on the mind. It is evident that the production of ideas in our minds can be no reason why we should suppose corporeal substances to exist, since the rise of those ideas is acknowledged to remain equally inexplicable with or without the supposition of material existences. In short, if there were external bodies, it is impossible that we should ever come to know it; and if there were not, we should have the same reasons to think there were, that we have now. We perceive a continual succession of ideas; some are anew excited, others are changed or totally disappear. There is, therefore, some cause of these ideas, whereon they depend, which produces and changes them. This cause must be a substance; but it has been shown that there is no corporeal or material substance. It remains, therefore, that the cause of ideas is an incorporeal active substance or spirit. A spirit is one simple, undivided, active being; as it perceives ideas it is called the "understanding," and as it produces or otherwise operates about them, it is called the "will." Such is the nature of spirit that it cannot be of itself perceived, but only by the effects which it produceth. The ideas of sense are more strong, lively, and distinct than those of the imagination; they have likewise a steadiness, order, and coherence, and are excited in a regular series, the admirable connection whereof sufficiently testifies the wisdom and benevolence of its Author. The set rules or established methods, wherein the mind that we depend on excites in us the ideas of sense, are called the "laws of Nature." These we learn by experience, and so obtain a sort of foresight which enables us to regulate our actions for the benefit of life. In general, to obtain such or such ends such or such means are conducive; and all this we know, not by discovering any necessary connection between our ideas, but only by the observation of the laws of Nature.