Not impossible, but it would have been harder. If you did succeed in thinking about them, it seems likely that along the way you would invent a language that supported them. By that I mean you would have developed notation to write your thoughts down compactly. You wouldn't need to implement the language.<p>There was nothing to stop people from doing calculus pre-Newton. But doing calculus without the language (theorems and definitions) of calculus would be hard. Once Newton had provided the language, it became something high school students could learn.<p>(Historical accuracy not guaranteed, but I hope it illustrates my point.)