When Dr. Johnson talks about philosophy, he's referring to knowledge in general rather than a specific branch. In this particular discussion, he observes that many men retreat from the hurly-burly of society, with all its hustle and bustle, into a world of tranquil solitude, where, far from the madding crowd, they hope to be able to acquire knowledge. This is what Johnson means when he says that some men consider solitude to be the parent of philosophy.

Johnson agrees with such people, but only up to a point. He readily concedes that solitude can indeed confer learning. However, whatever learning it does confer is completely useless unless it's taught to others. In other words, at some point, the solitary scholar will have to emerge from the cramped confines of his study and re-enter the world of men and affairs, the very world from which he originally escaped in search of solitude.

Furthermore, Johnson argues, the acquisition of knowledge often comes about through conversation with others. It is in learned society, the kind of society of which the gregarious Dr. Johnson was himself a member, that men often learn to develop their knowledge.

For the most part, this is because, in conversation with others, men will meet with objections to their ideas, which can help them discover their errors or any faulty reasoning in their arguments. This simply wouldn't be possible were they to have remained in solitude, hidden away in their studies.