Scientific realism says that the entities, states and processes described by correct theories really do exist. Protons, photons, fields of force, and black holes are as real as toe-nails, turbines, eddies in a stream, and volcanoes. The weak interactions of small particle physics are as real as falling in love. Theories about the structure of molecules that carry genetic codes are either true or false, and a genuinely correct theory would be a true one.

Even when our sciences have not yet got things right, the realist holds that we often get close to the truth. We aim at discovering the inner constitution of things and at knowing what inhabits the most distant reaches of the universe. Nor need we be too modest. We have already found out a good deal.

Anti-realism says the opposite: there are no such things as electrons. Certainly there are phenomena of electricity and of inheritance but we construct theories about tiny states, processes and entities only in order to predict and produce events that interest us. The electrons are fictions. Theories about them are tools for thinking. Theories are adequate or useful or warranted or applicable, but no matter how much we admire the speculative and technological triumphs of natural science, we should not regard even its most telling theories as true. Some anti-realists hold back because they believe theories are intellectual tools which cannot be understood as literal statements of how the world is. Others say that theories must be taken literally – there is no other way to understand them.