When Keats lamented that scientific inquiry would ‘conquer all mysteries by rule and line’ and ‘unweave a rainbow’, he was, it turns out, being unduly pessimistic. Science has revealed a vista of new wonders and brought us up sharply against the limits of what we can know. Consider, for example, as Adam Nicolson does at one point in his wonderfully wide-ranging new book, the work of Benoît Mandelbrot, the father of the fractal, who demonstrated that, in Nicolson’s words, ‘the closer you look at something, the more it remains unknown. Knowledge cannot embrace whatever it seeks to know. It can only sit alongside the world, contingent, touching it, maybe, at one or two points but shrinking beside the unaddressable and limitless actuality of things.’ Such a simple question as ‘How long is the coast of Britain?’ has no answer once you start looking closely at the actuality.

The coast of Britain – specifically the intertidal shoreline of a bay on the Morvern peninsula on the west coast of Scotland – is where this book begins and ends. Here Nicolson, fascinated by the animal and plant life revealed by the changing tides, sets out to build three rock pools and see what turns up in them. To the Victorians, rock pools were ‘gardens of prelapsarian bliss’. To Nicolson, they are ‘one of the most revelatory habitats on earth’.

He begins by looking closely at five life forms that appear in his pools: sandhopper, prawn, winkle (whose Latin name, Littorina littorea, means ‘shorey shore-thing’), crab and anemone. Nicolson brings each of these vividly to life, discussing everything from the sandhopper’s array of specialised legs, ‘like a multi-bladed penknife’, to the delicate and dangerous copulation of crabs, a process that can last for days. Along the way, startling facts come up: tiny sandhoppers, for example, are so keen on chewing things up that they can reduce a single plastic bag to about 1.75 million fragments of toxic microplastic (a very good reason not to leave litter on the beach). But Nicolson’s mind is forever roaming beyond the narrow confines of biology to consider such questions as whether a prawn is a machine or a being with a self. And he ponders the apparent calm of life in a pool of prawns, concluding that it is ‘in reality a version of rigidified terror’. This is an idea that leads into the final chapter of the first section of the book, ‘Heraclitus on the Shore’, a lucid exposition of the pre-Socratic philosopher’s idea of the world as liquid, in a state of constant flux, its apparent stability the result of forces straining against each other and finding balance. By this light, the rock pool is a perfect Heraclitean microcosm.

As the book proceeds, the vision gets ever broader as Nicolson considers planetary forces and the dizzying notion of deep time and explores the human history of the Morvern shoreline, from the Mesolithic period to the pre-modern Gaelic world, in which humans and animals were ‘co-actors in the drama of existence’ and the ‘still folk’ (‘fairies’ or ‘little people’) walked the earth. A Gaelic word, dùthchas, describes this sense of the unity of land, people, nature, culture and all living things. In the end, the dùthchas of this coastal community proved too fragile to withstand the coming of the modern world: it withered away just at the moment when the Victorian love affair with rock pools began. Nicolson’s philosophical reflections lead him to ponder Heidegger’s ideas of ‘total thereness’ and ‘being-with’, and the Kantian notion of noumena (‘things-in-themselves’), before, in his concluding section, he returns to the bay and sets about building another rock pool.

This is a richly satisfying book, a worthy successor to Nicolson’s great study of seabirds, The Seabird’s Cry. The Sea is Not Made of Water is beautifully written and driven always by the author’s endless curiosity, his breadth of knowledge and his sense of the mystery and wonder of the world.