OBSERVATION AND EXPERIMENT. 439 



be rules for observing. But these, like rules for 

 inventing, are properly instructions for the prepara- 

 tion 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 strengthening 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 ana- 

 lysis, depend upon 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 should 

 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 age when men believed 

 in the occult influences of the heavenly bodies, it 

 might have been unphilosophical to omit ascertaining 

 the precise condition of those bodies at the moment of 

 the experiment. As to the degree of minuteness of 

 the mental sub-division ; if we were obliged to break 

 down what we observe into its very simplest elements, 

 that is, literally into single 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 unne- 

 cessary. 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 



