Different modes of motion have long been known as the causes of our different colour perceptions, or at any rate as associated therewith, and of late years, more especially since the promulgation of Newlands’ {260a} law, it has been perceived that what we call the kinds or properties of matter are not less conditioned by motion than colour is. The substance or essence of unconditioned matter, as apart from the relations between its various states (which we believe to be its various conditions of motion) must remain for ever unknown to us, for it is only the relations between the conditions of the underlying substance that we cognise at all, and where there are no conditions, there is nothing for us to seize, compare, and, hence, cognise; unconditioned matter must, therefore, be as inconceivable by us as unmattered condition; {261a} but though we can know nothing about matter as apart from its conditions or states, opinion has been for some time tending towards the belief that what we call the different states, or kinds, of matter are only our ways of mentally characterising and docketing our estimates of the different kinds of motion going on in this otherwise uncognisable substratum. Our conception, then, concerning the nature of any matter depends solely upon its kind and degree of unrest, that is to say, on the characteristics of the vibrations that are going on within it. The exterior object vibrating in a certain way imparts some of its vibrations to our brain—but if the state of the thing itself depends upon its vibrations, it must be considered as to all intents and purposes the vibrations themselves—plus, of course, the underlying substance that is vibrating. If, for example, a pat of butter is a portion of the unknowable underlying substance in such-and-such a state of molecular disturbance, and it is only by alteration of the disturbance that the substance can be altered—the disturbance of the substance is practically equivalent to the substance: a pat of butter is such-and-such a disturbance of the unknowable underlying substance, and such-and-such a disturbance of the underlying substance is a pat of butter. In communicating its vibrations, therefore, to our brain a substance does actually communicate what is, as far as we are concerned, a portion of itself. Our perception of a thing and its attendant feeling are symbols attaching to an introduction within our brain of a feeble state of the thing itself. Our recollection of it is occasioned by a feeble continuance of this feeble state in our brains, becoming less feeble through the accession of fresh but similar vibrations from without. The molecular vibrations which make the thing an idea of which is conveyed to our minds, put within our brain a little feeble emanation from the thing itself—if we come within their reach. This being once put there, will remain as it were dust, till dusted out, or till it decay, or till it receive accession of new vibrations.