Put very crudely and perhaps over simply, the answer to this conundrum, at the end of Bion's journey, turned out something like this.
The group, any group, organisation, society, needs and evolves a structure of tasks, roles, procedures, rules, ascribed status (what Bion referred to as the "group culture"), in order to contain the anxiety of the unknown and the responses which, unconsciously, are mobilised to defend against that unknown. The unknown is at the same time what is unknown and feared in each of us, and what is unknown in the realities we engage with as we live and work.
Within the group, Bion believed, one can see operating a number of powerful unconscious and unlearned, quasi-instinctive, strategies of evasion and denial. Bion came to see these strategies of evasion and denial as constituting what he termed a "group mentality", opposed to the conscious aims, intentions and efforts of individuals. No group, no organisation, and no individual; however sophisticated, is ever wholly outside the sphere of group mentality in this sense. And, paradoxically, Bion believed, this is at least partly because the ability of the group to mobilise group mentality is a powerful, unlearned source of co-operation, (Valency) Of course, co-operation has more benign roots than this as well. We need to co-operate to achieve many if not all of our individual desires and aims. Just as, and this is a point Bion never denied, we need structures through which to co-operate and give direction to the enterprises we engage in. But these more positive, and more reality based sources of co-operation and structure always as it were "ride on the back" of something far more primitive and defensive. The task of leadership and the intuitive skill of the gifted leader is to balance the requirements of co-operation and structure in the service of reality, with the constraints inevitably imposed by group mentality in the service of defence.
I have talked about 'group mentality' here in very general terms. In fact, much of Bion's pioneering work lay in his charting the variety of strategies of evasion and denial which comprised that mentality. There were three such strategies which Bion identified, each of which was mutually exclusive, though interchangeable, and each of which was characterised by particular constellations of emotion and fantasy. Bion called these "Basic Assumptions": baslc, because they seemed to be rudimentary, unlearned, instinctive; assumptions, because they operated like myths on the basis of an implicit "as if". They were named respectively as Dependence, Fight/Flight and Pairing.
Briefly, a group operating under a basic assumption of dependence behaves "as if it is met in order to be sustained by a leader on whom it depends for nourishment, material and spiritual, and protection". The leader may be a person, a book or set of ideas and beliefs which operates as a bible.
A group operating under Fight/Flight behaves as if it has met to fight something or run away from it.
A group operating under basic assumption Pairing is preoccupied with the idea of the potential birth or emergence of someone or something that will save it from its present state, from feelings of hatred, destructiveness and despair. Its prevailing emotionality is one of hopefulness and expectation: as Bion puts it, that the coming season will be more agreeable; that some new kind of community - an improved group, society, nation - will be developed etc.
It was important, for me anyway, to recognise that in this section Wilfred Bion was learning to not lead a group in the traditional sense. This would in itself be the traditional psychoanalytic stance between analyst and analysand, and one of the key reactions is that the analysand's anxiety is triggered by the analysts "absence."