On 23 January 1895, after his play Guy Domville had failed on the London stage, Henry James, in his early 50s, wrote in his Notebook: “I take up my old pen again – the pen of all my old unforgettable efforts and sacred struggles. To myself – today – I need say no more. Large and full and high the future still opens. It is now indeed that I may do the work of my life.”

I like these words. For anyone approaching middle age, or wading through it, they may be the most useful words anyone has written, words that if we repeat often enough we may even start to act upon. Words that could change our lives, or the long sweet stretch of it that is left.

Over the next six years James wrote a number of novels and stories. However, only one of them – The Turn of the Screw – matches earlier work such as Washington Square and The Portrait of a Lady. But what he was doing, or what his unconscious was doing, in those years as he wrote novels about English manners such as The Spoils of Poynton and What Maisie Knew, was collecting images for what would be one the most intense, valuable and rewarding short periods in the life of any artist. At the beginning of the 20th century, James published three great novels – The Wings of the Dove (1902), The Ambassadors (1903) and The Golden Bowl (1904).

While published second, The Ambassadors was written first. It tells the story of Lambert Strether arriving in Paris in middle-age from America to ensure the return home of his friend Mrs Newsome’s only son Chad, who has been lingering in the city and worrying his poor mother to death.

In England, Strether, who is in the company of Waymarsh, his flat-footed friend from home, meets the worldly and witty Maria Gostrey, who takes him in hand and who becomes a sort of novelist-within-the-novel, a figure who asks sharp questions and offers clever analysis.

At the opening of the book, as Strether prepares for his journey to Paris, James offers him “such a consciousness of personal freedom as he hadn’t known in years”. The novel, then, will become a drama about the conflict between restriction and personal liberty, and about the power of fresh experience to transform or indeed bewilder the self.

James, in all his work, treated personal liberty with immense care. He loved the openness of the imagination, but as a good son of New England he learned not to trust fully what he loved. When he played with notions of personal freedom, he always allowed limits to appear, or responsibilities. Thus while Strether becomes captivated himself by Paris, its beauty, the freedom it offers, the people he meets, the demands of Chad’s austere and worthy mother appear more pressing.

Strether notices that Chad has been transformed by the city, and especially by his association with the beguiling Madame de Vionnet and her daughter. It is part of James’s genius not to make Chad a mere spendthrift wanderer and not to make Madame de Vionnet, despite her allure, too exotic or strange. James drew his characters with a considerable amount of  shade.

Henry James’s father took his children from America to Europe because he was concerned that in America they were not getting a sufficiently sensuous education. The lesson of The Ambassadors comes when Strether says to a young friend of Chad’s: “Live all you can; it’s a mistake not to. It doesn’t matter so much what you do in particular as long as you have your life.”

The implications surrounding our responsibility to live, and live sensuously, are at the heart of The Ambassadors. James, however, was a novelist more than he was a teacher. His novels change our lives not for their telling us how to live, but for their enacting in a prose that is subtle and filled with nuance a drama replete with irony and pure clarity and then mystery. Nothing is simple in his world.

As Strether seems determined to notice everything, we learn in a brilliant scene that he has failed to see something essential, that in his hunger for fresh experience, his own innocence has emerged even more strongly.

Thus James, by his own great example, as he moved into his 60s, may show us how to keep the future “large and full and high”, as he wrote, and Strether, too, may in his response to fresh life show us that middle age may be more interesting and more open to suggestion than youth.

But the real way this novel changed my life was through its own complex textures, its own deep opposition to complacency, its own refusal to settle for anything small.

As we, the middle-aged ones, move along the last stretch, these may be the most important things to know and remember and believe in.

Colm Toíbín is Leonard Milberg lecturer in Irish letters at Princeton University. His novel Brooklyn was Costa novel of the year