Software does not make a computer a new machine

Analogy

If we have patented an automobile which can drive anywhere, we cannot then come back and file patents for driving from Albequerque to San Diego, etc. -- the more general patent already applies.

Argument

Similarly, the general purpose computer, which can peform any computation that is Turing complete, is already patented. Filing additional patents on particular subsets of that general computing ability of the computer is like filing for a patent on driving a car from one particular place to another -- it is a subset of the capabilities which have already been patented. It is in fact simply filing for a patent on a table of numbers which happens to make a given computer perform a given computation -- yet the general property of being able to perform any computation which can be expressed as a table of numbers has already been patented. Thus any patent on software is redundant.