Dear Brother, I cannot tell what is become of my labours among that people: If all that my Lord builded by me be caſten down, & the bottom fallen out of the profeſſion of that parish, […] how can I bear it?

...for those things that synecdochize this relationship between the real and the spectral.

The captain stood abaft the wheelhouse.

Category theory also contributes new proof techniques, such as diagram chasing or arguments by duality; Steenrod called these methods “abstract nonsense.”³ The aim of this text is to introduce the language, philosophy, and basic theorems of category theory. A complementary objective is to put this theory into practice: studying functoriality in algebraic topology, naturality in group theory, and universal properties in algebra.