The doctrine that certain truths about the physical world could be ‘known a priori’

 

"Another example was in geometry . For centuries, no clear distinction was made between its status as a mathematical system and as a physical theory – and at first that did little harm, because the rest of science was very unsophisticated compared with geometry , and Euclid’s theory was an excellent approximation for all purposes at the time. But then the philosopher Immanuel Kant (1724–1804), who was well aware of the distinction between the absolutely necessary truths of mathematics and the contingent truths of science, nevertheless concluded that Euclid’s theory of geometry was self -evidently true of nature. Hence he believed that it was impossible rationally to doubt that the angles of a real triangle add upto 180 degrees. And in this way he elevated that formerly harmless misconception into a central flaw in his philosophy , namely the doctrine that certain truths about the physical world could be ‘ known a priori ’ – that is to say , without doing
science. And of course, to make matters worse, by ‘ known ’ he unfortunately meant ‘justified’ ."