What an individual is capable of may be measured by how far his understanding is from his willing. What a person can understand he must also be able to make himself will. Between understanding and willing lie the excuses and evasions.

Søren Kierkegaard, The Journals (1846)

I read this SK quote online. So it does not have a detailed source attached to it – a personal gripe. But I like the last sentence: between thinking and doing are excuses. People know what the right thing to do is but they evade it with excuses (not reasons). It illustrates again that there is a qualitative difference between knowing and doing.