Andrea del Castagno began life as a farm lad, but was educated as an artist at the cost of one of the less notable Medici. He had a vigorous way with his brush, as we see here and have seen elsewhere. In the Duomo, for example, we saw his equestrian portrait of Niccolò da Tolentino, a companion to Uccello's Hawkwood. When the Albizzi and Peruzzi intrigues which had led to the banishment of Cosimo de' Medici came to their final frustration with the triumphant return of Cosimo, it was Andrea who was commissioned by the Signoria to paint for the outside of the Bargello a picture of the leaders of the insurrection, upside down. Vasari is less to be trusted in his dates and facts in his memoir of Andrea del Castagno than anywhere else; for he states that he commemorated the failure of the Pazzi Conspiracy (which occurred twenty years after his death), and accuses him not only of murdering his fellow-painter Domenico Veneziano but confessing to the crime; the best answer to which allegation is that Domenico survived Andrea by four years. We may now return to S. Marco. The convent as we now see it was built by Michelozzo, Donatello's friend and partner and the friend also of Cosimo de' Medici, at whose cost he worked here. Antonino, the saintly head of the monastery, having suggested to Cosimo that he should apply some of his wealth, not always too nicely obtained, to the Lord, Cosimo began literally to squander money on S. Marco, dividing his affection between S. Lorenzo, which he completed upon the lines laid down by his father, and this Dominican monastery, where he even had a cell reserved for his own use, with a bedroom in addition, whither he might now and again retire for spiritual refreshment and quiet. It was at S. Marco that Cosimo kept the MSS. which he was constantly collecting, and which now, after curious vicissitudes, are lodged in Michelangelo's library at S. Lorenzo; and on his death he left them to the monks. Cosimo's librarian was Tommaso Parenticelli, a little busy man, who, to the general astonishment, on the death of Eugenius IV became Pope and took the name of Nicholas V. His energies as Pontiff went rather towards learning and art than anything else: he laid the foundations of the Vatican library, on the model of Cosimo's, and persuaded Fra Angelico to Rome to paint Vatican frescoes.