An Essay Concerning Human Understanding By John Locke Book II: Of Ideas, Chapters 12-33

21. God immoveable because infinite.

If it be said by any one that it cannot change place, because it hath none, for the spirits are not IN LOCO, but UBI; I suppose that way of talking will not now be of much weight to many, in an age that is not much disposed to admire, or suffer themselves to be deceived by such unintelligible ways of speaking. But if any one thinks there is any sense in that distinction, and that it is applicable to our present purpose, I desire him to put it into intelligible English; and then from thence draw a reason to show that immaterial spirits are not capable of motion. Indeed motion cannot be attributed to God; not because he is an immaterial, but because he is an infinite spirit.

22. Our complex idea of an immaterial Spirit and our complex idea of Body compared.

Let us compare, then, our complex idea of an immaterial spirit with our complex idea of body, and see whether there be any more obscurity in one than in the other, and in which most. Our idea of BODY, as I think, is AN EXTENDED SOLID SUBSTANCE, CAPABLE OF COMMUNICATING MOTION BY IMPULSE: and our idea of SOUL, AS AN IMMATERIAL SPIRIT, is of A SUBSTANCE THAT THINKS, AND HAS A POWER OF EXCITING MOTION IN BODY, BY WILLING, OR THOUGHT. These, I think, are our complex ideas of soul and body, as contradistinguished; and now let us examine which has most obscurity in it, and difficulty to be apprehended. I know that people whose thoughts are immersed in matter, and have so subjected their minds to their senses that they seldom reflect on anything beyond them, are apt to say, they cannot comprehend a THINKING thing which perhaps is true: but I affirm, when they consider it well, they can no more comprehend an EXTENDED thing.

23. Cohesion of solid Parts in Body as hard to be conceived as thinking in a Soul.

If any one says he knows not what it is thinks in him, he means he knows not what the substance is of that thinking thing: No more, say I, knows he what the substance is of that solid thing. Further, if he says he knows not how he thinks, I answer, Neither knows he how he is extended, how the solid parts of body are united or cohere together to make extension. For though the pressure of the particles of air may account for the cohesion of several parts of matter that are grosser than the particles of air, and have pores less than the corpuscles of air, yet the weight or pressure of the air will not explain, nor can be a cause of the coherence of the particles of air themselves. And if the pressure of the aether, or any subtiler matter than the air, may unite, and hold fast together, the parts of a particle of air, as well as other bodies, yet it cannot make bonds for ITSELF, and hold together the parts that make up every the least corpuscle of that MATERIA SUBTILIS. So that that hypothesis, how ingeniously soever explained, by showing that the parts of sensible bodies are held together by the pressure of other external insensible bodies, reaches not the parts of the aether itself; and by how much the more evident it proves, that the parts of other bodies are held together by the external pressure of the aether, and can have no other conceivable cause of their cohesion and union, by so much the more it leaves us in the dark concerning the cohesion of the parts of the corpuscles of the aether itself: which we can neither conceive without parts, they being bodies, and divisible, nor yet how their parts cohere, they wanting that cause of cohesion which is given of the cohesion of the parts of all other bodies.

24. Not explained by an ambient fluid.

But, in truth, the pressure of any ambient fluid, how great soever, can be no intelligible cause of the cohesion of the solid parts of matter. For, though such a pressure may hinder the avulsion of two polished superficies, one from another, in a line perpendicular to them, as in the experiment of two polished marbles; yet it can never in the least hinder the separation by a motion, in a line parallel to those surfaces. Because the ambient fluid, having a full liberty to succeed in each point of space, deserted by a lateral motion, resists such a motion of bodies, so joined, no more than it would resist the motion of that body were it on all sides environed by that fluid, and touched no other body; and therefore, if there were no other cause of cohesion, all parts of bodies must be easily separable by such a lateral sliding motion. For if the pressure of the aether be the adequate cause of cohesion, wherever that cause operates not, there can be no cohesion. And since it cannot operate against a lateral separation, (as has been shown,) therefore in every imaginary plane, intersecting any mass of matter, there could be no more cohesion than of two polished surfaces, which will always, notwithstanding any imaginable pressure of a fluid, easily slide one from another. So that perhaps, how clear an idea soever we think we have of the extension of body, which is nothing but the cohesion of solid parts, he that shall well consider it in his mind, may have reason to conclude, That it is as easy for him to have a clear idea how the soul thinks as how body is extended. For, since body is no further, nor otherwise, extended, than by the union and cohesion of its solid parts, we shall very ill comprehend the extension of body, without understanding wherein consists the union and cohesion of its parts; which seems to me as incomprehensible as the manner of thinking, and how it is performed.

We can as little understand how the parts cohere in extension as how our spirits perceive or move.

25. I allow it is usual for most people to wonder how any one should find a difficulty in what they think they every day observe. Do we not see (will they be ready to say) the parts of bodies stick firmly together? Is there anything more common? And what doubt can there be made of it? And the like, I say, concerning thinking and voluntary motion. Do we not every moment experiment it in ourselves, and therefore can it be doubted? The matter of fact is clear, I confess; but when we would a little nearer look into it, both in the one and the other; and can as little understand how the parts of body cohere, as how we ourselves perceive or move. I would have any one intelligibly explain to me how the parts of gold, or brass, (that but now in fusion were as loose from one another as the particles of water, or the sands of an hour-glass,) come in a few moments to be so united, and adhere so strongly one to another, that the utmost force of men's arms cannot separate them? A considering man will, I suppose, be here at a loss to satisfy his own, or another man's understanding.

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