On this supposition, I, in the first place, described this matter, and essayed to represent it in such a manner that to my mind there can be nothing clearer and more intelligible, except what has been recently said regarding God and the soul; for I even expressly supposed that it possessed none of those forms or qualities which are so debated in the Schools, nor in general anything the knowledge of which is not so natural to our minds that no one can so much as imagine himself ignorant of it.
Euler is credited for popularizing the Greek letter π (lowercase pi) to denote the ratio of a circle’s circumference to its diameter, as well as first using the notation f(x) for the value of a function, the letter i to express the imaginary unit √-1, the Greek letter Σ (capital sigma) to express summations, the Greek letter Δ (capital delta) for finite differences, and lowercase letters to represent the sides of a triangle while representing the angles as capital letters.