This is not an easy book to read. The arguments presented are more like inside baseball, physicist to physicist, or something along that line. Three points however stuck out.
When it comes to mathematics, Penrose calls himself a Platonist. (1) By that he means that mathematical truths exist "out there," in an external, independent sense, governing the material world. (2) Penrose comes to this perspective, at least in part, because of certain problems - unresolvable problems - related to the connection between quantum and classical (gravity?) physics and the inability to account for or explain human consciousness. In view of these and other challenges, Penrose concludes that there must be a transcendent reality that the human mind apprends (ascends to), much as Plato argued. In other words, mathematical truth reflects Plato's realm and mathematics describes that realm for us. (3)
Regarding the mind's relationship with this Platonic world, Penrose's argument, along with the three respondents to it, was hard for me to grasp. I guess the classical view of mind is in terms of electrical impulses, moving linearly across synapses. Penrose might be, in contrast, indicating a belief that mind functions in a non-linear quantum sense. This is a whole new area for me. I don't know if he's talking about glial cells or the argument made by others about global "wave" communications within the brain, or conscious awareness communicated immediately by intuition. Penrose does note a certain continuity of mind through evolution but for him, the human mind, with consciousness, takes off and emancipates itself from the material world, leaving perception and stimulation way behind. It is this that allows for some sort of ascension into the Platonic realm.
It is of course fair game to critique existing paradigms of "knowing" for their various shortcomings or inadequacies, and it's fair game to speculate on alternative ways to see the nature of reality. But Penrose goes beyond this and asserts the existence of his transcendent world that is, in some fundamental Platonic sense, superior to and governs the physical world. That, it seems to me, is to travel beyond the headlights and to give up on the known, the physical world, and to put all the chips on something that is, minimally, highly speculative or, maximally, like the world of faith, on something that simply cannot be known. (4)
I thought that Hawking's response ("The Objections of an Unashamed Reductionist") made a good argument for sticking with the program - a this-world endeavor to explain reality. "We pretty much agree on the classical theory of General Relativity," Hawking writes, "but disagreements began to emerge when we go to to quantum gravity. We now have very different approaches to the world, physical and mental. Basically, he's a Platonist believing that there's a unique world of ideas that describes a unique physical reality. I, on the other hand, am a positivist who believes that physical theories are just mathematical models we construct...." He goes on to say that "The evolution from earthworm brains to human brains presumably took place by Darwinian natural selection. The quality selected for was the ability to escape enemies and to reproduce, not the ability to do mathematics." Mind's capacity to think abstractly, however that occurred, made humans more successful, and its by-product is the capacity to look at the world objectively, for mathematical and scientific truths, as verified by experience, testing, and peer review. Ironically, there may be inherent limits to that capacity to know the world objectively per Heisenberg's uncertainty principle. The quantum world at its heart is a "thing in itself," "independent," and perhaps eternal and absolute. It is not "out there." It is in this world, but forever beyond our capacity to know.
(1) The term "Platonist" is interesting. Penrose refers to the allegory of the cave that illustrates the illusory nature of the physical world. That is his only direct reference to Plato's writing that, taken as a whole in Plato's some twenty-two dialogues, indicates that Plato's transcendent reality - I realize this is not the standard view - was religious in nature, with it's eternal notions taken, perhaps, from Indian thought. Whether Penrose is referring to Plato's world, or to the world of Plato's successors who might interpret Plato in a non-religious sense, is not clear.
(2) In contrast to those who think that mathematical concepts are "merely as idealizations of our physical world...emerging from the world of physical objects," most mathematicians Penrose states think of our physical world "as a structure precisely governed according to timeless mathematical laws. Thus, they prefer to think of the physical world, more appropriately, as emerging out of the ('timeless') world of mathematics."
(3) Penrose also writes that "One can well take the view that the 'Platonic world' contains other absolutes, such as the Good and the Beautiful," and he appears to lean in this direction as well, but his focus in this book is only on "the Platonic concepts of mathematics."
(4) Of course, it may be a question of interpretation. What does 'independent," or "out there" mean? However these might be understood, Penrose is explicit in his opening chapter that it is, distinctly, not of this world. It is a world of "absolutes" - but where or how it exists is unknown, and particularly so since Einstein ruled out notions of absolute space and time. Maybe it is this that Penrose is hitting, dead on: Even if space-time is not absolute, there is still something out there, fixed and eternal, which can be grasped by human consciousness.