2. For this purpose, I remark that the Colligation of ascertained Facts into general Propositions may be considered as containing three steps, which I shall term the _Selection of the Idea_, _the Construction of the Conception_, and _the Determination of the Magnitudes_. It will be recollected that by the word _Idea_, (or Fundamental Idea,) used in a peculiar sense, I mean certain wide and general fields of intelligible relation, such as Space, Number, Cause, Likeness; while by _Conception_ I denote more special modifications of these ideas, as a _circle_, a _square number_, a _uniform force_, a _like form_ of flower. Now in order to establish any law by reference to facts, we must select the _true Idea_ and the _true Conception_. For example; when Hipparchus found[25\3] that the distance of the bright star Spica Virginis from the equinoxial point had increased by two degrees in about two hundred years, and desired to reduce this change to a law, he had first to assign, if possible, the _idea_ on which it depended;--whether it was regulated for instance, by _space_, or by _time_; whether it was determined by the positions of other stars at each moment, or went on progressively with the lapse of ages. And when there was found reason to select _time_ as the regulative _idea_ of this change, it was then to be determined how the change went on with the time;--whether uniformly, or in some other manner: the _conception_, or the rule of the progression, was to be {188} rightly constructed. Finally, it being ascertained that the change did go on uniformly, the question then occurred what was its _amount_:--whether exactly a degree in a century, or more, or less, and how much: and thus the determination of the _magnitude_ completed the discovery of the law of phenomena respecting this star. [Note 25\3: _Hist. Ind. Sc._ b. iii. c. iv. sect. 3.]