Our conviction then of the objective reality of matter (at least from the point of view of the Natural Philosopher) is based upon the experimental truth that we can neither increase nor diminish its quantity, in fact on what we may conveniently for our present purpose call the _Conservation of Matter_. 95. Here let us pause for a moment to compare together this view of matter and the definition of the laws of the universe, which we have already given. The laws of the universe we defined (Art. 54) to be the laws according to which the beings in the universe are trammelled by the Governor thereof as regards time, space, and sensation. Now, it may be asked, is this definition consistent with a belief in the objective reality of matter? Our reply is, that to our minds the two are in perfect accordance. We do not here intend to enter into any metaphysical discussion. It is enough for us to say that our practical working certainty of the reality of matter depends upon the facts, _firstly_, that it offers resistance to our imagination and our will, and, _secondly_, that in particular it offers _absolute_ resistance to all attempts to change its quantity. We shall soon see that experiment teaches us that both properties belong to something else. 96. Returning from this digression let us therefore assume that the objective reality of the external universe has been proved, and that this reality is strongly impressed upon us in virtue of that principle which we have called the conservation of matter. But as soon as we grant this, we are obliged by our reason, however little our senses may incline us to it, or rather however much they may dispose us against it, to allow objective reality to whatever else may be found to be _in the same sense_ conserved. (We have here italicised these four words for a reason which will afterwards appear.) This is a question which deserves and must secure careful consideration. 97. In abstract dynamics several things are said and mathematically proved by deductions from experiment to be conserved, but one only of these in the strict sense in which we have spoken of the conservation of matter. We will examine them briefly, and our non-mathematical readers must pardon us if we make use of certain technical expressions belonging to the domain of mathematical physics.