* Idea: A thought or suggestion.
* Imagination: The ability to form mental images.
* Importance: The state of being important or essential.
* Independence: The state of being free from control or influence.
* Integrity: The quality of being honest and having strong moral principles.
* Intelligence: The ability to learn and understand.
* Injustice: The state of being unfair or wrong.
* Insanity: The state of being mentally ill.
* Inspiration: The process of being mentally stimulated to do or feel something.
* Interest: A feeling of wanting to know more about something.
* Intuition: The ability to understand something instinctively.
* Isolation: The state of being alone or separated from others.
* Issue: A matter of public concern or debate.
These are just a few examples. The specific abstract nouns that start with "I" will depend on the context.