Open source is a vital step in moving software engineering from arcane art to science. Three hundred years ago, alchemy became science when its practitioners abandoned secrecy to embrace process transparency and peer review. Today software engineering is undergoing a similar transition, moving from closed to open source development. Only when we complete this transition, adopting open source development as a normal practice, can software development assume its place as a mature engineering discipline. Both evidence and theory confirm that open source delivers better reliability, lower costs, shorter development times and high quality of code (including better security).