He was infirm of body but still keen of mind, and though it looked like he couldn't walk across the room, he crushed me in debate.
The thought is that you see an episode of observation, experiment, or reasoning as confirming or infirming a hypothesis depending on whether the probability of it being correct increases or decreases during the episode.
sick weakly frail feeble enfeebled unhealthy debilitated weak decrepit ill
and to old or of in persons state health