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I'm confused by something super simple. When taking functional variation (e.g. of the action) in the context of field theory, I often see $$ {\delta \phi(x) \over \delta \phi(y)} = \delta(x-y) \ .$$ How does this make dimensional sense when the LHS naively appears dimensionless, while the RHS has mass dimension 1?

Qmechanic
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Rudyard
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