Let us now praise, admire, commend, extol, honour, eulogize, congratulate and applaud Peter Mark Roget, the patron saint of synonyms.

But let us first tackle the 150-year-old debate over whether Roget's Thesaurus, which first appeared in May 1852, is the most useful book ever written or, conversely, a blight on the language that has enabled countless lazy writers to bulk up their prose with words they barely understand and immediately forget.

A thesaurus may be both these things. At its worst, it is a crutch, for crossword enthusiasts, students desperate to imply a little learning in an essay crisis, headline writers, nervous after-dinner speakers and, yes, journalists. At its best, a thesaurus can jog out of the memory a word that would otherwise remain lost.

Critics sniffed at Roget's Thesaurus from the start. “Its practical utility, we think, is overrated,” declared Harper's Magazine. The London Critic agreed, insisting that the vast compilation of words and their close relations was “not likely to be so practically useful as the care, and toil, and thought bestowed upon it might have deserved”. How wrong they were: Roget's Thesaurus has sold upwards of 35 million copies; every edition sells more than the last.

Roget is known for this one book, written as a diversion and published late in life, but in a way his entire life was itself a homage to words, their richness, strangeness and therapeutic power.

Roget was born in London's Soho to a French Huguenot father and Swiss mother. The early death of his father and a beloved grandfather left Roget on the edge of a lifelong depression, which he salved with words: he gathered, nurtured and hoarded them. He grew into an astonishing polymath and an obsessive list-maker. Taking his cue from the great taxonomist Carl Linneaus, who codified and classified plants and animals, Roget set about organising words as a way, perhaps, of holding back the chaos he feared in his own personality, and the mental illness that ran in his family.

Roget is remembered for his book of words, but he was interested in everything. A doctor by training, and an intellectual magpie by inclination, his work on the persistence of vision (the way an image briefly lasts on the retina) would lead, eventually, to the invention of cinema. He studied Dante, water purification, dinosaur bones, phrenology and insects. He worked, lucky man, at the Pneumatic Institution for Inhalation Gas Therapy, where Coleridge came to breathe nitrous oxide. He helped to create the slide rule and London's sewerage system. He invented the first travel chess set.

The words also poured out of him: 300,000 for the Encyclopaedia Britannica, including the entries for “Ant” and “Bee”; his 250,000-word Bridgewater Treatise, was an attempt to systematise the animate world into four branches of physiology.

At the same time, he gathered words, their synonyms, their siblings, their cousins, correlatives and opposites in a vast family tree of vocabulary. He did this for half a century, starting when he was 21, in private, for his own personal pleasure and self-improvement...“Conceiving that such a compilation might help to supply my own deficiencies...I often found this little collection, scanty and imperfect though it was, of much use to me in literary composition.” He was 73 years old when he finally decided to publish the fruits of this hobby.

Roget had no time for writers who “indulge in the habit of arbitrarily fabricating new words and newfangled phraseology...in the illegal mint of their own fancy.” Yet his thesaurus (a word he invented) is today at least 20 times as long as the one he published, thanks to the never-ending evolution of words and phrases.

The word-gatherer made no claim to be a great writer. He was merely fascinated by the interconnectedness of words, and brilliantly realised that the more words available, the broader the possibilities of language. Some people are lucky enough to be able to find the mot juste without ever needing to consult those parallel words. For the rest of us, who find that the right word is sometimes lurking just out of reach, there is Roget and his thesaurus.

Of course, some use the thesaurus as a cheap supermarket, simply whipping off the shelves the first halfway decent substitute word they see. One can usually spot the thesaurus-abusers, for they tend to reverse George Orwell's rules in Politics and the English Language: using two words where one will do, (one syllable bad, many syllables good), and deploying a £5 word when an honest and serviceable penny word is to hand.

But, well used, Roget is a synonym for lucidity. At its best, his thesaurus is a reminder of what words can do, a testament to the rich density of the language itself.

There is something refreshingly honest about Roget, a man who knew so much about so much, yet whose greatest contribution to literature was the understanding that no one can hold an entire dictionary head in his head. Roget's Thesaurus should be used as a memory refresher, to keep the language forever young.

That, perhaps, is what Peter Pan means in J.M.Barrie's classic, when he remarks of Captain Hook: “The man is not wholly evil - he has a Thesaurus in his cabin.”

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