Deleuze and Guattari are not writing their book WHAT IS PHILOSOPHY? out of old age as an empirical reality, but in relation to the spectre of old age, which can sometimes and in certain cases accord a « moment of grace between life and death ». Such moments of grace, whenever they occur, are in a complex relation with time, death, and the multiplicity of spirits which haunt us.
Derrida, supposedly a thinker that does not assert anything clearly and unambiguously, is quite affirmative on this point:
It can only happen, if it – learning how to live – remains to be done, between life and death. Neither in life nor in death alone. What happens between two, and between all the » two ‘s » one likes, such as between life and death, can only maintain itself with some ghost, can only converse with or about some ghost. So it would be necessary to learn spirits (SPECTRES OF MARX, translation modified).
Deleuze and Guattari speak more concretely here. Learn spirits (what Deleuze and Guattari call « spiritual entities »), means also learn their concepts, personae, landscapes, diagrams and planes.
But how can we « learn spirits »? One answer is to learn philosophy. Philosophy is one long conversation with spirits, with and by means of our own spirit.