For some reason I was wondering if the people who invented irrigation should be considered intellectuals. I dont remember why.

There are 3 ways this question can be interpreted and answered:

1. What are the marks of an influential intellectual? These are the types of intellectuals that have made the biggest impact in their field of study. Without them, many succeeding intellectuals wouldn't have existed(For example, we could argue that if Plato was never born, the whole history of western philosophy would be radically different. Of course, it could of been better or worse without Plato so we can't assume his impact was positive necessarily.)

The marks of an influential intellectual seem to be:
-Creativity/Originality
-Charisma
-Determination
-Confidence
-Sometimes Arrogance
-Disagreeableness
-Willingness to face persecution for one's beliefs

2. What are the marks of a competent/skilled intellectual? These are intellectuals that have the greatest capacity to think. You can think of them as a kind of virtuoso in thinking. They can maybe write a complicated book in like a couple of weeks or have extensive knowledge in a variety of disciplines. They can understand incomprehensible concepts and come up with complicated ideas. They might employ skilled rhetoric also. Although, despite their skill in thinking, they might not make any meaningful contribution to the history of human though.

The marks of a competent intellectual are:
-Extreme Intelligence
-Great memory and fast thinking
-Interest in a variety of subjects
-Obsession with thinking for its own sake
-Having nothing better to do with their time other than working on intellectual pursuits.

3. What are the marks of an Admirable/Praiseworthy intellectual? These are intellectuals worthy of respect regardless of what opinion you have of their ideas. It is the spirit and the attitude by which they think that makes them special. I think Socrates, in many ways, fits this ideal. He has a genuine curiosity and a desire to learn and understand. For him, thinking isn't about promoting your ideas or showing how smart you are, it is about trying to discover the truth together.

The marks of an admirable intellectual are:
-Humility
-Intellectual Honesty
-Curiosity
-Desire to understand
-Agreeableness
-Good listening skills