If it be a quality superadded merely to the ordinary powers of matter, then it must exist equally in every part of the mass to which it is attached; every particle of the matter in which it inheres must be sentient, intelligent, voluntary, and active; and, on this supposition, it will remain a difficult, if not desperate problem, to account for the _unity_ of consciousness by such a diversity of parts, and especially for the _continuity_ of consciousness, when the material elements are confessedly in a state of constant flux and mutation. It would seem, too, that if thought be thus connected with an extended, divisible, and mutable substance, it must be itself extended, and, of course, divisible; and, accordingly, Dr. Priestley does not hesitate to affirm that our _ideas_, as well as our _minds_, possess these characters. "Whatever ideas," he says, "are in themselves, they are evidently produced by external objects, and must therefore correspond to them; and since many of the objects or archetypes of ideas are _divisible_, it necessarily follows that _the ideas themselves are divisible also_." ... "If the archetypes of ideas have _extension_, the ideas which are expressive of them, and are actually produced by them according to certain mechanical laws, must have extension likewise; and, therefore, the mind in which they exist, whether it be material or immaterial, must have extension also.... I am, therefore, obliged to conclude that the sentient principle in man, containing ideas which certainly have parts, and are divisible, and consequently must have extension, cannot be that simple, indivisible, and immaterial substance that some have imagined it to be, but something that has real extension, and therefore may have the other properties of matter."[169] He argues that _ideas_ must be extended and divisible because their objects or archetypes are so; and, further, that the _mind_ itself must be material, because these properties belong to the ideas which inhere in it as their subject or seat. Now, _this_ argument is fairly met by the reasoning, or the ridicule, call it which you will, of Dr. Thomas Brown: "In saying of mind that it is matter, we must mean, if we mean anything, that the principle which thinks is hard and divisible; and that it will be not more absurd to talk of the _twentieth_ part of an affirmation, or the _quarter_ of a hope, of the _top_ of a remembrance, and the north and east corners of a comparison, than of the twentieth part of a pound, or of the different points of the compass, in reference to any part of the globe. The true answer to the statement of the Materialist,--the answer which we feel in our hearts, on the very expression of the plurality and divisibility of feeling,--is that it assumes what, far from admitting, we cannot even understand, and that, with every effort of attention which we can give to our mental analysis, we are as incapable of forming any conception of what is meant by the quarter of a doubt, or the half of a belief, as of forming to ourselves an image of a circle without a central point, or of a square without a single angle."[170]