Ultimately, the Open Source movement is an extension of the scientific method, because at the heart of the computer industry lies computer science. To me programming is more than an important practical art. It is also a gigantic undertaking in the foundations of knowledge. Computer science, though, differs fundamentally from all other sciences. To demonstrate the validity of a program to someone, you must provide them with the means to compile and run the program.
Replication makes scientific results robust.
One scientist cannot expect to account for all possible test conditions, nor necessarily have the test environment to fully test every aspect of a hypothesis. By sharing hypotheses and results with a community of peers, the scientist enables many eyes to see what one pair of eyes might miss. By sharing source code, Open Source developers make software more robust. Programs get used and tested in a wider variety of contexts than one programmer could generate, and bugs get uncovered that otherwise would not be found. Because source code is provided, bugs can often be removed, not just discovered, by someone who otherwise would be outside the development process.