One very important circumstance of agreement in the physical investigations of mind and matter, we found to be, that, of both matter and mind, the successive phenomena are all which we truly know, though by the very constitution of our nature, it is impossible for us not to ascribe these to some _permanent subject_. _Matter_ is the permanent subject of certain qualities, extension, and its consequent divisibility, attraction, repulsion; that is to say, it is the permanent exhibiter to us of certain varying phenomena which we observe. _Mind_ is the permanent subject of certain qualities or states or affections of a different class--perception, memory, reason, joy, grief, love, hate; that is to say, of certain varying phenomena of which we are conscious. What _matter_ is independent of our perception; what _mind_ is independent of its temporary variety of feeling, it is impossible for us to discover; since whatever new knowledge of matter we can suppose ourselves to acquire, must be acquired by our perception, and must, therefore, be relative to it; and whatever new knowledge we can suppose ourselves to acquire of mind, must be itself a state or affection of the mind, and, therefore, only a new mental phenomenon to be added to those with which we were before acquainted, as one of the many states in which the permanent substance _mind_ is capable of existing. Since it is only by their relation to our own feelings, then, that substances can be known to us, beyond these relations it would be vain for us to think of penetrating; as vain, at least, as would be the attempts of the deaf to discover, by a process of reasoning, the nature of the sensations of sound, or of the blind to determine, not the lines of direction merely, in which the various coloured rays of light pass after refraction, for these they may optically determine, but the various sensations, corresponding with all the varieties of tint into which the sun-beams are broken by the drops of a falling shower. The substance matter, the substance mind, are, in this respect, to the whole race of metaphysical inquirers, what the rainbow, as a series of colours, is to opticians, who have never seen.