Karl Popper 
The Logic of Scientific Discovery [PDF ebook] 

Described by the philosopher A.J. Ayer as a work of »great originality and power», this book revolutionized contemporary thinking on science and knowledge. Ideas such as the now legendary doctrine of »falsificationism» electrified the scientific community, influencing even working scientists, as well as post-war philosophy. This astonishing work ranks alongside The Open Society and Its Enemies as one of Popper»s most enduring books and contains insights and arguments that demand to be read to this day.