'Consider this list of scientific controversies: the decision by the International Astronomical Union to define 'planet' in such a way as to exclude Pluto; the ongoing dispute over what 'species' means for bacteria; and the discovery, initially controversial, that the most common cause for peptic ulcers is an infection by the Helicobacter pylori bacterium. What do these diverse issues have in common? They are all examples of scientists disagreeing over how to classify phenomena 'correctly'. Categorization is a fundamental skill learned in childhood. Yet the principles that guide it are sometimes misunderstood and misused by scientists. Here we analyse the concepts of 'category' and 'class' and reveal that some controversies over scientific classification, such as the case of planets, should not be controversial at all. Others, including the case of ulcers, can be explained as a consequence of wrong assumptions about categories.'