Definition 1.4.20 (Natural transformations). If \(C\) and \(D\) are \(\infty \)-categories, we define a natural transformation of functors \(C \to D\) to be a morphism in \(\Fun (C,D)\). By (un)currying, this may equivalently be encoded as a functor \([1] \times C \to D\). We denote the hom anima in \(\Fun (C,D)\) by \(\Nat (f,g)\): \[ \Nat (f,g) := \Hom _{\Fun (C,D)}(f,g). \]
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