This is the Erasmus by Hans Holbein the Younger, painted in 1523. Notice how the artist has surrounded the sitter with items that reflect his interests and profession - this was an idea developed further in another famous work of Holbein's, The Ambassadors. If you look closely at the book on the back shelf, you'll see a Latin couplet praising Holbein's skill. It's thought to be written by Erasmus himself, where he says it's easier to criticize Holbein than to emulate him.
The use of everyday objects in the painting is also worth noting - this was an innovative technique at the time, and really adds to the sense of depth and detail. The sitter himself, Erasmus, was a prominent scholar and writer, and it's clear that Holbein has tried to capture his essence here.
Erasmus hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.