“His idea was to begin with those broad truths that must underlie all conceivable mental existences and establish a basis on those. The great principles of geometry, to begin with. He proposed to take some leading proposition of Euclid’s, and show by construction that its truth was known to us, to demonstrate, for example, that the angles at the base of an isosceles triangle are equal, and that if the equal sides be produced the angles on the other side of the base are equal also, or that the square on the hypotenuse of a right-angled triangle is equal to the sum of the squares on the two other sides. By demonstrating our knowledge of these things we should demonstrate our possession of a reasonable intelligence.⁠ ⁠… Now, suppose I⁠ ⁠… I might draw the geometrical figure with a wet finger, or even trace it in the air.⁠ ⁠…”