Quasicategories as a concrete model

Warning 1.6.16. Readers who prefer a concrete model may henceforth think of \(\infty \)-categories as quasicategories. This should only be used as a conceptual aid: statements that refer to the underlying simplicial set, such as equality of vertices or horn fillers, are statements about the model rather than about the abstract \(\infty \)-category. In the rest of the book we therefore continue to formulate everything model-independently.