[Beckett] deftly suppressed his philosophical reading in public statements. We don’t know why. According to David Addyman and Matthew Feldman between 1930 and 1938 he wrote 266 Folios of five hundred sides of variously typed and handwritten recto and verso notebook pages on philosophy taken from four sources. […] For this reason it is not unlikely that Beckett was not always sketching a generalized philosophical theme but had a specific one in mind.
Thomas Dilworth and Christopher Langlois think so. They propose that when Hamm remembers visiting a ‘madman’ in some anomalous visit, a recollection occurring halfway through ‘Endgame’ that draws attention to both memory and its defaults as well as the strategic location of the occurrence in the text as a whole, the ‘madman’ is to be identified as Nietzsche. […]
They conclude that, as with allusions to Hegel’s ‘Phenomenology of Spirit’ in the characters Pozzo and Lucky of ‘Godot’, Beckett ‘… is repeating the strategy of referring through dramatized imagery to a famous philosophical text in order to emphasise a pervasive theme in the play’. Jean-Michel Rabaté notes that Beckett’s relationship to philosophy is purposively playful. At times it works as shorthand, a gesture towards some idea that may or may not be a central concern, at other times it illustrates some more general feature, such as the structure of dramatic reality in the use of the sorites found also in ‘Endgame’.
Was Beckett a philosopher? (He is not in The IP Bible). Richard Marshall in 3:AM Magazine outlines parallels between Nietzsche and Beckett. In this extract he argues that Beckett systematically uses philosophy in his work.