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

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.
