Why do some sources call calculus, "the calculus"?

Question

No need to cite specific sources since I think it's a fairly common thing to see. What's up with that?

Thank you

Edit: I've seen it in several places. Here's where I'm currently looking at it at: Calculus: An Intuitive and Physical Approach (Second Edition): Here on amazon. Read the sample provided by Amazon and you'll see it several times in the first chapter.

Answer 1

Technically, "a calculus" is just any method or system of calculation involving some symbolic manipulation. For example, the Feynman calculus for computing amplitudes in quantum field theory, propositional calculus for formulas in logic, or the calculus of variations for maximizing and minimizing functionals. "The" calculus is usually reserved for that of Newton and Leibniz, the study of limiting procedures of differentiation and integration, and is almost always just called "calculus".

Answer 2

Because 'calculus' meant a set of rules for calculating things whereas 'the calculus' meant 'the infinitesimal calculus'. The qualifier was lost in the academic war over the foundations of the subject.