He is a positivist who thinks based on scientific evidence.

He considers himself an existentialist.

He is a true pragmatist, valuing practice over theory.

He claims to be a true democrat at heart.

He is a true optimist, always believing in a bright future.

My sister is a vegetarian and does not eat meat at all.

He is known as an absolutist in politics.

He is a fervent communist.

She is looking for an ideal partner.

The law of ideal gas is a basic concept in physics.