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Chapter IV
Idea of Solidity
1. We receive this idea from touch. The idea of solidity we
receive by our touch: and it arises from the resistance which we
find in body to the entrance of any other body into the place it
possesses, till it has left it. There is no idea which we receive more
constantly from sensation than solidity. Whether we move or rest, in
what posture soever we are, we always feel something under us that
support us, and hinders our further sinking downwards; and the
bodies which we daily handle make us perceive that, whilst they remain
between them, they do, by an insurmountable force, hinder the approach
of the parts of our hands that press them. That which thus hinders the
approach of two bodies, when they are moved one towards another, I
call solidity. I will not dispute whether this acceptation of the word
solid be nearer to its original signification than that which
mathematicians use it in. It suffices that I think the common notion
of solidity will allow, if not justify, this use of it; but if any one
think it better to call it impenetrability, he has my consent. Only
I have thought the term solidity the more proper to express this idea,
not only because of its vulgar use in that sense, but also because
it carries something more of positive in it than impenetrability;
which is negative, and is perhaps more a consequence of solidity, than
solidity itself. This, of all other, seems the idea most intimately
connected with, and essential to body; so as nowhere else to be
found or imagined, but only in matter. And though our senses take no
notice of it, but in masses of matter, of a bulk sufficient to cause a
sensation in us: yet the mind, having once got this idea from such
grosser sensible bodies, traces it further, and considers it, as
well as figure, in the minutest particle of matter that can exist; and
finds it inseparably inherent in body, wherever or however modified.
2. Solidity fills space. This is the idea which belongs to body,
whereby we conceive it to fill space. The idea of which filling of
space is,- that where we imagine any space taken up by a solid
substance, we conceive it so to possess it, that it excludes all other
solid substances; and will for ever hinder any other two bodies,
that move towards one another in a straight line, from coming to touch
one another, unless it removes from between them in a line not
parallel to that which they move in. This idea of it, the bodies which
we ordinarily handle sufficiently furnish us with.
3. Distinct from space. This resistance, whereby it keeps other
bodies out of the space which it possesses, is so great, that no
force, how great soever, can surmount it. All the bodies in the world,
pressing a drop of water on all sides, will never be able to
overcome the resistance which it will make, soft as it is, to their
approaching one another, till it be removed out of their way:
whereby our idea of solidity is distinguished both from pure space,
which is capable neither of resistance nor motion; and from the
ordinary idea of hardness. For a man may conceive two bodies at a
distance, so as they may approach one another, without touching or
displacing any solid thing, till their superficies come to meet;
whereby, I think, we have the clear idea of space without solidity.
For (not to go so far as annihilation of any particular body) I ask,
whether a man cannot have the idea of the motion of one single body
alone, without any other succeeding immediately into its place? I
think it is evident he can: the idea of motion in one body no more
including the idea of motion in another, than the idea of a square
figure in one body includes the idea of a square figure in another.
I do not ask, whether bodies do so exist, that the motion of one
body cannot really be without the motion of another. To determine this
either way, is to beg the question for or against a vacuum. But my
question is,- whether one cannot have the idea of one body moved,
whilst others are at rest? And I think this no one will deny. If so,
then the place it deserted gives us the idea of pure space without
solidity; whereinto any other body may enter, without either
resistance or protrusion of anything. When the sucker in a pump is
drawn, the space it filled in the tube is certainly the same whether
any other body follows the motion of the sucker or not: nor does it
imply a contradiction that, upon the motion of one body, another
that is only contiguous to it should not follow it. The necessity of
such a motion is built only on the supposition that the world is full;
but not on the distinct ideas of space and solidity, which are as
different as resistance and not resistance, protrusion and not
protrusion. And that men have ideas of space without a body, their
very disputes about a vacuum plainly demonstrate, as is shown in
another place.
4. From hardness. Solidity is hereby also differenced from hardness,
in that solidity consists in repletion, and so an utter exclusion of
other bodies out of the space it possesses: but hardness, in a firm
cohesion of the parts of matter, making up masses of a sensible
bulk, so that the whole does not easily change its figure. And indeed,
hard and soft are names that we give to things only in relation to the
constitutions of our own bodies; that being generally called hard by
us, which will put us to pain sooner than change figure by the
pressure of any part of our bodies; and that, on the contrary, soft,
which changes the situation of its parts upon an easy and unpainful
touch.
But this difficulty of changing the situation of the sensible
parts amongst themselves, or of the figure of the whole, gives no more
solidity to the hardest body in the world than to the softest; nor
is an adamant one jot more solid than water. For, though the two
flat sides of two pieces of marble will more easily approach each
other, between which there is nothing but water or air, than if
there be a diamond between them; yet it is not that the parts of the
diamond are more solid than those of water, or resist more; but
because the parts of water, being more easily separable from each
other, they will, by a side motion, be more easily removed, and give
way to the approach of the two pieces of marble. But if they could
be kept from making place by that side motion, they would eternally
hinder the approach of these two pieces of marble, as much as the
diamond; and it would be as impossible by any force to surmount
their resistance, as to surmount the resistance of the parts of a
diamond. The softest body in the world will as invincibly resist the
coming together of any other two bodies, if it be not put out of the
way, but remain between them, as the hardest that can be found or
imagined. He that shall fill a yielding soft body well with air or
water, will quickly find its resistance. And he that thinks that
nothing but bodies that are hard can keep his hands from approaching
one another, may be pleased to make a trial, with the air inclosed
in a football. The experiment, I have been told, was made at Florence,
with a hollow globe of gold filled with water, and exactly closed;
which further shows the solidity of so soft a body as water. For the
golden globe thus filled, being put into a press, which was driven
by the extreme force of screws, the water made itself way through
the pores of that very close metal, and finding no room for a nearer
approach of its particles within, got to the outside, where it rose
like a dew, and so fell in drops, before the sides of the globe
could be made to yield to the violent compression of the engine that
squeezed it.
5. On solidity depend impulse, resistance, and protrusion. By this
idea of solidity is the extension of body distinguished from the
extension of space:- the extension of body being nothing but the
cohesion or continuity of solid, separable, movable parts; and the
extension of space, the continuity of unsolid, inseparable, and
immovable parts. Upon the solidity of bodies also depend their
mutual impulse, resistance, and protrusion. Of pure space then, and
solidity, there are several (amongst which I confess myself one) who
persuade themselves they have clear and distinct ideas; and that
they can think on space, without anything in it that resists or is
protruded by body. This is the idea of pure space, which they think
they have as clear as any idea they can have of the extension of body:
the idea of the distance between the opposite parts of a concave
superficies being equally as clear without as with the idea of any
solid parts between: and on the other side, they persuade themselves
that they have, distinct from that of pure space, the idea of
something that fills space, that can be protruded by the impulse of
other bodies, or resist their motion. If there be others that have not
these two ideas distinct, but confound them, and make but one of them,
I know not how men, who have the same idea under different names, or
different ideas under the same name, can in that case talk with one
another; any more than a man who, not being blind or deaf, has
distinct ideas of the colour of scarlet and the sound of a trumpet,
could discourse concerning scarlet colour with the blind man I
mentioned in another place, who fancied that the idea of scarlet was
like the sound of a trumpet.
6. What solidity is. If any one ask me, What this solidity is, I
send him to his senses to inform him. Let him put a flint or a
football between his hands, and then endeavour to join them, and he
will know. If he thinks this not a sufficient explication of solidity,
what it is, and wherein it consists; I promise to tell him what it is,
and wherein it consists, when he tells me what thinking is, or wherein
it consists; or explains to me what extension or motion is, which
perhaps seems much easier. The simple ideas we have, are such as
experience teaches them us; but if, beyond that, we endeavour by words
to make them clearer in the mind, we shall succeed no better than if
we went about to clear up the darkness of a blind man's mind by
talking; and to discourse into him the ideas of light and colours. The
reason of this I shall show in another place.
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