Full Idea
It follows from the nature of a triangle that its three angles are equal to two right angles.
Gist of Idea
The sum of its angles follows from a triangle's nature
Source
Baruch de Spinoza (The Ethics [1675], IV Pr 57)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.204
A Reaction
This is the essentialist view of mathematics, which I take to be connected to explanation, which I take to be connected to the direction of explanation.