The maturity of knowledge

The ultimate result of thinking for yourself is what Schopenhauer calls ‘the maturity of knowledge’, which is a state of total organic integration between thoughts and experiences. This maturity does not have to do with the power of one's intellect but the organisation of its contents. 

Thinking is, at the least, looking at things for yourself. It avoids putting a foreign concept between the mind and the world, otherwise the two will not make contact. Schopenhauer ensured that anything he found in books was assimilated only if he could connect it to experience.