The branching idea is similar to Hugh Everett III's "many-worlds" hypothesis that the collapse of the quantum mechanical wave function can be eliminated (Dirac's projection postulate ignored) and thus physical determinism could be restored to physics. Everett had met with Einstein at Princeton and Einstein had long hoped for a return to deterministic physics.

McCall's version does not branch at quantum experiments by physicists (Everett's idea is absurdly anthropomorphic). McCall says his branching points have always been in existence, pre-determined since the beginning of the universe. This idea flies in the face of the lack of information at the earliest times in the universe.

Let us take stock. I am suggesting that the brain be regarded as possessing two levels of functioning, a lower indeterministic level that creates choice-sets, and a higher more organized and structured level, deterministic or 99 per cent deterministic, that embodies rational choice. Because of its basic lower-level indeterminism the overall functioning of the brain is not deterministic: identical inputs into identical states of the brain do not necessarily yield identical outputs. The option that is unerringly selected by the higher-level process may be the option that is probabilistically the least likely, measured by branch proportionality. The indeterministic factor is essential, since if it were not there the number of objectively possible courses of action open to us at any given moment would be limited to one. Although the range of choice is generated by chance, the acted-upon alternative is not selected by chance. It is selected in accordance with a rational procedure, based on goals, objectives, rules, likes, dislikes, memories, passions, etc. Whatever the eventual chosen course of action, there will in general be an intentional explanation for it. In this way the brain embodies a process of rational choice, built upon an indeterministic option-generating foundation.

Many years ago, Karl Popper wrote a long article in which he argued that the idea of a being that could predict its own future states involved a contradiction. The purpose of the article was to demonstrate that universal Laplacian determinism was impossible in a universe containing sufficiently powerful predictors. In this chapter we have been concerned not with beings that can predict their states, but with beings that can select their own states. Our emphasis has been on action, not knowledge. But the same general conclusion seems to apply: beings that can do this can exist only in an indeterministic universe, such as the universe of the branched model. In fact they do it in virtue of being themselves indeterministic, thereby being capable of generating choice-sets, and then exercising prohairesis. Such entities, of which our brains are a prime example, may correctly be described as indeterministic mechanisms which select their own future states, in accordance with a goal-directed or rule-governed decision procedure.

(vi) Summing-up

The question at issue is this: can a theory of free and responsible action be built upon indeterministic assumptions? In the context of the branched model, how is it possible consciously and deliberately to guide the first branch point into one particular area of the model, avoiding others? I hope that by now an answer to these questions will have emerged, in outline at least.

A philosopher sits at a table writing, and a deliberative question occurs to him. Should he continue writing, or should he break off, put the kettle on, and make himself a cup of coffee? These two alternatives are located on different sets of future branches, in different areas of the universe tree. It seems presumptuous to think that the philosopher, by his own bare choice, should have the power to direct the first node of the model into one region rather than another. Yet this is so. In the first place, the philosopher is not like a deterministic Turing machine. The quantum- based indeterministic functioning of his brain guarantees the existence of a choice-set containing at least two options, both of which are physically possible. Secondly the philosopher is not like a chess-playing machine equipped with a randomizer: the selection of one of these options will not be by chance. The philosopher deliberates, and makes up his mind to forget about coffee and continue working for another hour. The explanation-reason for this decision is that he wants to finish the book he is writing. The philosopher, an intelligent deliberator, has exercised his ability to represent alternative courses of action, evaluate them, and choose one. In the example just given there were two deliberation- reasons and one explanation-reason. It is not necessary to believe that the eventual choice was the result of anything but a deterministic or quasi-deterministic rational decison procedure. The philosopher's family could have predicted his decision with a high degree of accuracy. But the explanation of his choice is still a reason, not a cause. It cannot be a deterministic cause because of the antecedent existence of the choice-set, and it cannot be a probabilistic cause because of the phenomenon of change of mind, and because an alternative of low probability can still be chosen.

In the branched model, indeterminism plays two different roles. First, the mode! is indeterministic because of its branched structure. A deterministic model would be unbranched. Secondly, the selection of one branch out of indenumerably many above a node is itself random and indeterministic. That is to say, it is random and indeterministic unless an intelligent deliberator is present. Such deliberators, who have the power of prohairesis, constrain the indeterminism of the branched model by providing intentional explanations for branch attrition. Their powers are admittedly limited, but they do account for some tiny fraction of the global phenomenon of branch attrition. However, they do nothing to touch the branching character of the structure itself, without which there would be no intelligent choice and no explanations. Only within the context of indeterminism do deliberation, decision, and freedom assume their true shape, guided by rational or quasi-rational procedures and undistorted by deterministic fetters.

(A Model of the Universe, pp.277-279)

McCall and Lowe show "that libertarianism is a consistent philosophical thesis." They draw out the notion of an instantaneous choice (which compatibilists often attack as necessarily either determined or random, according to the standard argument against free will) into a continuous temporal process of deliberation that culminates in the decision.

They locate the indeterminism in the early part of deliberation, as do all two-stage models of free will. The decision itself they say is caused not by chance, but by the character and reasons of the agent.

McCall and Lowe are correct that both van Inwagen and Robert Nozick locate the indeterminism in the wrong place, namely the decision itself.

Leading libertarian philosopher Robert Kane also locates indeterminism in the choice, but Kane argues that in a "torn decision" all of the alternative possibilities for action can be independently defended by reasons, so the agent can take responsibility, whatever the particular choice.

McCall and Lowe extend van Inwagen's "replay" example by considering Kane's description of a decision as a temporal process:

To illustrate the model of decision-making we have in mind, we replace van Inwagen’s Alice by Robert Kane’s more temporally-extended example of Jane. Jane is deliberating whether to spend her vacation in Hawaii or Colorado. She takes her time, consults travel books and brochures, contemplates her bank account, and eventually comes to the conclusion that all things considered, Hawaii is the best option. At the end, she seals her decision by buying an air ticket to Honolulu. A useful way of analyzing this deliberative process (Aristotle’s bouleusis) is to divide Jane’s decision-making into three stages (McCall (1999)):

• (i) Choice-set formation (in Jane’s case identifying Hawaii and Colorado as her two options),
• (ii) Evaluation (weighing the reasons pro and con Hawaii against the reasons pro and con Colorado),
• (iii) Choice (Aristotle’s prohairesis).

A necessary requirement of indeterministic decision-making is that each option in the choice-set remain open, i.e. choosable, through the entire deliberation, right up to the moment of choice.

(Philosophy and Phenomenological Research, Vol. LXX, No. 3, May 2005, p.683)

McCall and Lowe summarize the many steps they see in their libertarian deliberative process:

The main features of the indeterministic deliberative process which demonstrates consistency are as follows.

(1) An agent X is faced with deciding between options A, B, C, ... [these options may involve chance and are not pre-determined.]

(2) There are, in X’s estimation, reasons for and reasons against each option.

(3) X uses her power of rational judgement to weight these reasons and to weigh one option against another.

(4) The process of weighing and weighting is controlled by X’s judgement, is on-going throughout the deliberation, and is justifiable to a third party.

(5) Each option remains open (choosable) up to the moment of decision.

(6) The deliberation ends with X’s reasoned choice of one of the options.

Conclusion: Rational, indeterministic, controlled deliberative processes prove that the concept of libertarian free will is internally consistent.

(Philosophy and Phenomenological Research, p.689)

Later, McCall and Lowe defended their indeterministic free will model against the Luck Objection. (Philosophy and Phenomenological Research, Vol. LXXVII No. 3, November 2008, p.745)