All that I will say in what follows depends on looking at mathematics as a human activity or a product of it, and at the philosophy of mathematics as a critical assessment of this activity and of its products. In particular, I look at mathematical theories as pieces of mathematics and, then, in turn, as products of human activity. These products result from a complex intellectual process, typically involving several subjects that communicate to each other in various forms (also through different ages) by using appropriate languages (mostly, but not only, written ones), possibly supplemented by suitable non-linguistic devices (such as diagrams or forms of ostension).