He was extraordinarily intelligent - he spoke seven languages - and had impressive organisational skills. We know this from Otto Sperling, who visited Rubens's studio in 1621.
The young Dane expresses a mix of surprise and admiration. He encountered Rubens while he was painting and being read to from the works of the Roman author Tacitus. He was also dictating a letter in the meantime as well as engaging in a conversation with Sperling.