The answer author rambled on without well structured paragraphs, but the author seems to have a correct intuition for it, since they wrote that "f(x)" and "df/d

The answer author rambled on without well structured paragraphs, but the author seems to have a correct intuition for it, since they wrote that "f(x)" and "df/dx" are not well-formed function identifiers (free variables), but "(df/dx)(x)" is OK (bound variables.