At the head of the chain of ideas are the "original sources." These are people -- like Plato, Descartes, Voltaire, Einstein -- who had breakthrough ideas, or who shaped the world through their work. Their history and works are usually so specialized that only experts can understand them. They are revered by many, even though few people try to read their original words.
We idealize these people. Often, they stood on the shoulders of those who came before them, and some benefited from the historical circumstances of their times. Their mistakes have been forgotten by history, and their triumphs have been exaggerated. Their names have been used by others to make money and gain power. Meanwhile, many "original sources" alive today are unrecognized or undervalued.
There's no substitute for reading or studying these "original sources." But make sure you read their own words; don't trust the hype of a biographer, or a hagiographer. Don't be surprised if you find that s/he has been misrepresented by the people who claim to follow their ideals!