 we are not possess’d of any instrument or art of measuring, which can secure us from all error and uncertainty… we therefore suppose some imaginary standard of equality, by which the appearances and measuring are exactly corrected, and the figures reduc’d entirely to that proportion… But tho’ this standard be only imaginary, the fiction however is very natural; nor is any thing more usual, than for the mind to proceed after this manner with any action, even after the reason has ceas’d, which first determin’d it to begin. This appears very conspicuously with regard to time; where tho’ ’tis evident we have no exact method of determining the proportions of parts, not even so exact as in extension, yet the various corrections of our measures, and their different degrees of exactness, have given us an obscure and implicit notion of a perfect and entire equality. The case is the same in many other subjects. A painter forms the same fiction with regard to colours. A mechanic with regard to motion. (THN 1.2.4.24)