The n -point correlation function ⟨ O 1 ( x 1 ) ⋯ ⋯ O n ( x n ) ⟩ is a function of the positions x i and other parameters of the fields O 1 , … … , O n . In the bootstrap approach, the fields themselves make sense only in the context of correlation functions, and may be viewed as efficient notations for writing axioms for correlation functions. Correlation functions depend linearly on fields, in particular ∂ ∂ x 1 ⟨ O 1 ( x 1 ) ⋯ ⋯ ⟩ = ⟨ ∂ ∂ x 1 O 1 ( x 1 ) ⋯ ⋯ ⟩ .