13. The secondary qualities of things not discovered by demonstration.
Not knowing, therefore, what number of
particles, nor what motion of them, is fit to produce any precise degree of whiteness, we cannot demonstrate the
certain equality of any two degrees of whiteness; because we have no certain standard to measure them by, nor
means to distinguish every the least real difference, the only help we have being from our senses, which in this
point fail us. But where the difference is so great as to produce in the mind clearly distinct ideas, whose
differences can be perfectly retained, there these ideas or colours, as we see in different kinds, as blue and red, are
as capable of demonstration as ideas of number and extension. What I have here said of whiteness and colours, I
think holds true in all secondary qualities and their modes.
