Some people exhibit an amazing lack of interest in reality, content to imagine living in a wholly invented world. The notion that much of subjective experience is illusory is strongly connected with the beginnings of “modern” philosophy.
Galileo and Locke claimed that only things which are physical and measurable really exist. Galileo argued that primary qualities; solidity, motion, figure, extension and number were really real – being the objective properties of objects and that secondary qualities; color, sight, sound, small, taste and touch did not actually exist per se. They are merely artifacts; products of the sense organs that really have nothing to do with the objects being perceived. They are merely what our brains do when confronted with sensory input and primary qualities.