The British progressive rock band Pink Floyd has been one of the most successful acts in the history of rock/pop. During the past 50 years they have sold tens of millions of albums and sold out several massive gigs. They are going to have their own special exhibition in the Victoria and Albert Museum in London, a testimony to their success. The history of the band, however, has been far from happy.

The breakdown of their first lead guitar player and songwriter, Syd Barrett, to mental illness had a profound effect on the band. Mental illness is reflected in the lyrics of several of their songs. In ‘If’ from the album Atom Heart Mother they sing: ‘If I go insane, please don't put your wires in my brain’. The song ‘Brain Damage’, from the multi-platinum album Dark Side of The Moon, is explicitly about mental illness. There are several references in the album The Wall, for example to derealisation in ‘Comfortably Numb’. But the impact of Barrett on the band is never more profound and explicit than on their album Wish You Were Here, arguably their best. The song ‘Shine on You Crazy Diamond’ is a tribute to their former band leader. It is one of their most personal and poignant songs, musically and lyrically. The description of Syd Barrett's eyes is one of the best lines in this song: ‘Now there's a look in your eyes, like black holes in the sky’.

In the documentary Pink Floyd and Syd Barrett Story, Roger Waters, who wrote lyrics to the song, explains the change in Syd's eyes as he became more unwell, which inspired the lyrics. The documentary shows several portrait pictures of Barrett where one can see the transition. Seeing the documentary made us think of the information we gather by observing our patients. Such information, although not part of the criteria of diagnosing mental illness, provides us with a unique insight to our patients' lives. This kind of evidence is complementary to our formal assessments, but it cannot be measured. The Austrian philosopher Ludwig Wittgenstein, in his monumental work Philosophical Investigations, calls it ‘imponderable evidence’. Skilfulness in eliciting such evidence is important for a psychiatrist, something we should not lose in the day-to-day, bureaucratic work.