(47.) By the former example it appears that the deflecting cause would have put a quiescent body in motion, and by the latter it would have reduced a moving body to a state of rest. Hence the phenomenon of a change of direction is to be referred to the same class as the change from rest to motion, or from motion to rest. The quality of inertia is, therefore, inconsistent with any change in the direction of motion which does not arise from an external cause. (48.) From all that has been here stated, we may infer generally, that an inanimate parcel of matter is incapable of changing its state of rest or motion; that, in whatever state it be, in that state it must for ever continue, unless disturbed by some external cause; that if it be in motion, that motion must always be _uniform_, or must proceed at the same rate, equal spaces being moved over in the same time: any increase of its rate must betray some impelling cause; any diminution must proceed from an impeding cause, and neither of these causes can exist in the body itself; that such motion must not only be constantly at the same uniform rate, but also must be always in the same direction, any deflection from one uniform direction necessarily arising from some external influence. The language sometimes used to explain the property of inertia in popular works, is eminently calculated to mislead the student. The terms resistance and stubbornness to move are faulty in this respect. Inertia implies absolute passiveness, a perfect indifference to rest or motion. It implies as strongly the absence of all resistance to the reception of motion, as it does the absence of all power to move itself. The term _vis inertiæ_ or _force of inactivity_, so frequently used even by authors pretending to scientific accuracy, is still more reprehensible. It is a contradiction in terms; the term _inactivity_ implying the absence of all force. * * * * * (49.) Before we close this chapter, it may be advantageous to point out some practical and familiar examples of the general law of inertia. The student must, however, recollect, that the great object of science is generalisation, and that his mind is to be elevated to the contemplation of the _laws_ of nature, and to receive a habit the very reverse of that which disposes us to enjoy the descent from generals to particulars. Instances, taken from the occurrences of ordinary life, may, however, be useful in verifying the general law, and in impressing it upon the memory; and for this reason, we shall occasionally in the present treatise refer to such examples; always, however, keeping them in subservience to the general principles of which they are manifestations, and on which the attention of the student should never cease to be fixed.