But the doctrine that time alone is not a cause of change of velocity in any body is further recommended to us by this consideration;--that time is conceived by us not as a cause, but only as a condition of other causes producing their effects. Causes operate in time; but it is only when the cause exists, that the lapse of time can give rise to alterations. When therefore all external causes of change of velocity are supposed to be removed, the velocity must continue identical with itself, whatever the time which elapses. An eternity of negation can produce no positive result. Thus, though the discovery of the First Law of Motion was made, historically speaking, by means of experiment, we have now attained a point of view in which we see that it might have been certainly known to be true independently of experience. This law in its ultimate form, when completely simplified and steadily contemplated, assumes the character of a self-evident truth. We shall find the same process to take place in other instances. And this feature in the progress of science will hereafter be found to suggest very important views with regard both to the nature and prospects of our knowledge. 3. _Gravity is a Uniform Force._--We shall find observations of the same kind offering themselves in a manner more or less obvious, with regard to the other principles of Dynamics. The determination of the laws according to which bodies fall downwards by the common action of gravity, has already been noticed in the History of Mechanics[24\3], as one of the earliest positive advances in the doctrine of motion. These laws were first rightly stated by Galileo, and {241} established by reasoning and by experiment, not without dissent and controversy. The amount of these doctrines is this: That gravity is a uniform accelerating force; such a _uniform force_ having this for its character, that it _makes the velocity increase in exact proportion to the time of motion_. The relation which the spaces described by the body bear to the times in which they are described, is obtained by mathematical deduction from this definition of the force. [Note 24\3: _Hist. Ind. Sc._ b. vi. c. ii. sect. 2.]