It's Clear Why Software Patents Need to Disappear | PCWorld Business Center:

1. Software Is Just Math

It is not possible to patent a mathematical concept, and for the very same reasons it should not be possible to patent software. Why? Because all software is fundamentally just a series of mathematical operations that are performed on inputs to generate results, whether a numerical value, text on a page or music from your speakers.

Just as it would be ridiculous to grant a patent on the algorithm for calculating “pi,” for example, so it is equally ridiculous to allow patents on software, all of which can be reduced to a set of comparable mathematical algorithms. The fact that such patents are routinely granted is a huge signal that those in charge of granting patents don't really understand what it is being patented.