There are very few people to whom the word polymath might genuinely be applied, but Raymond Tallis  doctor, gerontologist, philosopher, literary critic and novelist  is one. This collection of essays supplies what has been wanting for some time: an introduction, for the non-specialist reader, that gives some indication of the breadth and depth of his work. In Defence of Wonder is an apt title. What unites the various aspects of Tallis's thinking is a profound reaction against reductionism. Whether he is writing on the "neuromania" that insists the brain is identical to the mind, or the nature of time, or skewering the improbabilities of Ian McEwan's Saturday, Tallis seems allergic to forms of thought that deny the fine-grained, paradoxical and profuse nature of reality.

Neuromania was the subject of Tallis's last work, Aping Mankind, and parts of In Defence of Wonder expand on the fallacies he finds in a purely materialist account of the mind. What is so impressive is seeing the manner in which he mounts a double attack, taking on the claim that "feelings, emotions and thoughts are reducible to nerve impulses" as both bad science and bad philosophy. There is, I think, more that can be done with the arguments that Tallis sets out. Leibniz's "identity of indiscernibles"  the idea that "to suppose two things indiscernible is to suppose the same thing under two names"  should be brought into play whenever an over-enthusiastic neuroscientist says that the pain in your toe, when you kick a stone in an attempt to refute Bishop Berkeley's theory of perception, is identical with the transfer of a sodium ion deep in the intricate mesh and mush of your brain.

Tallis is also a wit (there's a thesis to be written on the great comedian philosophers, from Socrates to Hume to Kierkegaard to Derrida, though Tallis would balk at the last name). Writing of the experiment whereby Eric Kandel claimed to have found proof of memory through electrocuting sea-slugs, Tallis writes of the choice of subject: "First, it has relatively few neurons (20,000, compared with the hundreds of billions in your cerebral cortex alone); second, the neurons are strapping cables of a millimetre or more in diameter, and uniquely identifiable, so it is easy to see what is happening inside them and, more importantly, in their connections, the synapses. The beast also has the advantage of being ugly and dim and so it is not likely to attract the attention of the Animal Liberation Front or to seek legal advice."

Throughout the book, Tallis beguiles the reader, co-opts you to his wry perspective. A piece such as "An Introduction to Incontinental Philosophy" makes our capacity to pee or not to pee as great a conundrum as the more famous line he parodies, and makes a humanist case that the difference between humanity and the great apes is as evident in toilet rolls as in Beethoven quartets.

His work on tense and time raises radical issues. Tallis's express wish is to redeem time from physics  he might also have said, to rescue it from the exasperation of philosophers. Time, like many a little word, bundles a lot inside it. It can be reduced to a dimension in mathematics, but even then it is a dimension very unlike the other three: I can imagine rotating a cube on any of its three spatial axes, but rotating it on its temporal axis is just a piece of Doctor Who-speak. And what of the idea that any given moment in time, let's say 6 February 1970, is fretted into a problem if we think that it is in the past today, was the present on that day, and was in the future on 5 February 1970? The worry that the same thing cannot be "not yet", "now" and "no longer" underpins anxieties about tensed time from Aristotle's logical fatalism to JME McTaggart's 1908 paper "The Unreality of Time".

Tallis unpicks the problems with all the various philosophers' arguments, in particular with reference to possibilities  "call no event future until it is past" is his aphoristic solution  while offering other, deeper approaches to time. An alien with some unfeasibly powerful telescope might be able to watch the entire life of Napoleon unfold (this hypothetical alien is a simile for the fictitious, panoptic observer of a "block-universe"  one in which past, present and future co-exist). Said alien would know that Napoleon was on Elba on 20 January 1815. But it could not know that, on that day, Napoleon was thinking about the Battle of Borodino, which took place on 7 September 1812: as Tallis argues, tensed time exhibits both an imagined future and a regretted past. But it can be taken further. Napoleon could have been imagining what would have happened at Borodino if he had listened to his generals and brought the imperial guard into the battle. The alien cannot see the universe inside Napoleon's head. Tense is not just about time; it is analogous to Tallis's account of the visual field "dappled with visible invisibility", the way a woman in a bikini holding an umbrella in front of herself looks naked, if you have a dirty mind.

A central question in philosophy has been "why is there anything rather than nothing?". Tallis ponders the true wonder of a different question: "Why can we think about that which is not?" All this, and a wonderful meditation on why sex in literature is so bad: Tallis is not just a polymath, but a most congenial thinker with whom to exercise one's intellect.

 Stuart Kelly's The Book of Lost Books is published by Polygon.