It is well known that all modules over any ring have injective envelopes. On the other hand, the existence of projective covers of modules is quite rare and the corresponding rings were characterized by Bass in 1958. Over these rings projective covers coincide with flat covers. The existence of flat covers over a general ring was conjectured by Enochs in 1981 and proved recently by the works of Bican, Eklof, El Bashir, Enochs and Trlifaj. We would like to present the ideas of the proof using the language of purities.