Page:Herschel - A Preliminary Discourse on the Study of Natural Philosophy (1831).djvu/89

 

PART II.

Of the Principles on which Physical Science relies for its successful Prosecution, and the Rules by which a systematic Examination of Nature should be conducted, with Illustrations of their Influence as exemplified in the History of its Progress.

CHAPTER I.

Of Experience as the Source of our Knowledge.—Of the Dismissal of Prejudices.—Of the Evidence of our Senses

(66.) Into abstract science, as we have before observed, the notion of cause does not enter. The truths it is conversant with are necessary ones, and exist independent of cause. There may be no such real thing as a right-lined triangle marked out in space; but the moment we conceive one in our minds, we cannot refuse to admit the sum of its three angles to be equal to two right angles; and if in addition we conceive one of its angles to be a right angle, we cannot thenceforth refuse to admit that the sum of the squares on the two sides, including the right angle, is equal to the square on the side subtending it. To maintain the contrary, would be, in effect, to deny its being right angled. No one causes or makes all the diameters of an ellipse to be bisected in its