OBSERVATION AND EXPERIMENT. 273 



There may be rules for observing. But these, like rules for inventing, are 

 properly instructions for the preparation of one's own mind; for putting 

 it into the state in which it will be most fitted to observe, or most likely 

 to invent. They are, therefore, essentially rules of self-education, which is 

 a different thing from Logic. They do not teach how to do the thing, but 

 how to make ourselves capable of doing it. They are an art of strength- 

 ening the limbs, not an art of using them. 



The extent and minuteness of observation which may be requisite, and 

 the degree of decomposition to which it may be necessary to carry the 

 mental analysis, depend on the particular purpose in view. To ascertain 

 the state of the whole universe at any particular moment is impossible, but 

 would also be useless. In making chemical experiments, we do not think 

 it necessary to note the position of the planets; because experience has 

 shown, as a very superficial experience is sufficient to show, that in such 

 cases that circumstance is not material to the result : and accordingly, in 

 the ages when men believed in the occult influences of the heavenly bodies, 

 it might have been unphilosophical to omit ascertaining the precise condi- 

 tion of those bodies at the moment of the experiment. As to the degree 

 of minuteness of the mental subdivision, if we were obliged to break down 

 what we observe into its veiy simplest elements, that is, literally into sin- 

 gle facts, it would be difficult to say where we should find them ; we can 

 hardly ever affirm that our divisions of any kind have reached the ultimate 

 unit. But this, too, is fortunately unnecessary. The only object of the 

 mental separation is to suggest the requisite physical separation, so that 

 we may either accomplish it ourselves, or seek for it in nature ; and we 

 have done enough when we have carried the subdivision as far as the point 

 at which we are able to see what observations or experiments we require. 

 It is only essential, at whatever point our mental decomposition of facts 

 may for the present have stopped, that we should hold ourselves ready and 

 able to carry it further as occasion requires, and should not allow the free- 

 dom of our discriminating faculty to be imprisoned by the swathes and 

 bands of ordinary classification ; as was the case with all early speculative 

 inquirers, not excepting the Greeks, to whom it seldom occurred that what 

 was called by one abstract name might, in reality, be several phenomena, 

 or that there was a possibility of decomposing the facts of the universe into 

 any elements but those which ordinary language already recognized. 



§ 2. The different antecedents and consequents being, then, supposed to 

 be, so far as the case requires, ascertained and discriminated from one an- 

 other, we are to inquire which is connected with which. In every instance 

 which comes under our observation, there are many antecedents and many 

 consequents. If those antecedents could not be severed from one another 

 except in thought, or if those consequents never were found apart, it would 

 be impossible for us to distinguish {a posteriori at least) the real laws, or 

 to assign to any cause its effect, or to any effect its cause. To do so, we 

 must be able to meet with some of the antecedents apart from the rest, and 

 observe what follows from them ; or some of the consequents, and observe 

 by what they are preceded. We must, in short, follow the Baconian rule 

 of varying the circumstances. This is, indeed, only the first rule of phys- 

 ical inqiiiry, and not, as some have thought, the sole rule ; but it is the 

 foundation of all the rest. 



For the purpose of varying the circumstances, we may have recourse 

 (according to a distinction commonly made) either to observation or to ex- 



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