A fundamental law or primary source from which anything proceeds. He is a person of good principle. The ingenious scientist proved the fact thoroughly. Her excuses were ingenious. Her sincerity showed that she was ingenuous.
Ability is the power of accomplishing something.
Capacity is the power to receive training or instruction. You have the ability to use most of the information that you have the capacity to receive. He is a person of unusual capacity for acquiring knowledge, but with very little ability to make use of his knowledge. I know nothing of his character, but his reputation is good. There is less snow this year.