Sentence:  Thus, moreover, the principles of geometry- for example, that "in a triangle, two sides together are greater than the third," are never deduced from general conceptions of line and triangle, but from intuition, and this a priori, with <WSD>apodeictic</WSD> certainty. 
Meaning of the word marked with <WSD> tags:  Of the characteristic feature of a proposition that is necessary (or impossible): perfectly certain (or inconceivable) or incontrovertibly true (or false); self-evident.