The inability of computers to do math is not merely theoretical. It poses a major practical problem in getting computers to do what people want them to do. A handout called “Dangers of Computer Arithmetic,” from a computer science course at the Rochester Institute of Technology, for instance, notes several operations that are likely to cause problems, among them “adding quantities of very different magnitude” and “calculating the difference of two very large values.”
Great effort has been expended in hiding these realities from ordinary users. The impression given to casual users is that computer math just works. But the underlying reality of “just works” is a quite complicated substructure invented by clever humans, and reality sometimes slips through the cracks. Try typing “999,999,999,999,999 minus 999,999,999,999,998” into Google, for an illustration of how hazardous it is for a computer to calculate the difference of two very large values.