In art, portraits have had a poor time of it since the Second World War. Many came to think of portraiture as at best a minor form of art, inferior to the grander modernist traditions - such as minimalism, conceptual or action art - that ask the really important questions.
When you look at the art usually done under this label, the assessment seems accurate. After all, portraits are often largely painted to flatter the sitter, or at best to make observations about modern life and social attitudes. The latter kind would include artists like David Hockney or the (to my mind awful) portraits of the Glasgow Brutalist school.
But standing head and shoulders above all this are the portraits and especially the nudes of Lucian Freud. Here you are invited to look and look deeply at the person being painted. There is no flattery, no voyeurism. You could say that these pictures are cold and clinical, but for me the more you look the more you see. They cause you to question what beauty is.
Freud took his sitters out of their social class and context so that you see them as a person equal to you, to the artist and all others - though the titles of his paintings often do the opposite and emphasise it. In his eyes as the painter, and yours as the viewer, all are painted as equals, usually passive and open to view in order to show how people really are without artifice or social pretence. The paintings attempt to show you the truth - and it is the wonder of his artistry that this lofty claim to truth seems credible.
After his death a famous DJ and promoter of world music was asked what he thought about Freud's work. He said art was not his area but wasn't he the bloke who painted fat ugly women? That is like saying Al Green is a singer of soppy love songs - the point is not what he painted but how he painted. By painting the way he did he transformed his subject matter. His work provokes you to question what beauty is and what it means to be human. But mostly Freud wants you to look long and deeply, to really see the world. He was a great painter.
A special display of some of Lucian Freud's work can be seen at Tate Britain until 25 September. Free entry.