David Hume made some very telling comments on the problems we face in getting to grips with new ideas. In trying to explain his seemingly counter-intuitive ideas about ‘induction’ – the way we infer effects from causes, he suggested that we struggle to grasp new ideas, and quickly slip back into old ways of thinking because of the force ‘custom or habit’ on our minds. He said that we are;

inclined to behave or think in some way, not because it can be justified by reasoning or some process of the understanding but just because we have behaved or thought like that so often in the pass.

Thinking in a new way requires effort and concentration, and once we let that effort drop the old way of thinking seems to take over again. It is perhaps a bit like trying to swim against the current of a fast-flowing river: if we swim really hard we can make progress, but as soon as our work rate decreases the current quickly sweeps us back in the opposite direction. 

Whilst we are in the midst of reading and thinking about new ideas we can understand them, but once we ‘relax’, the understanding fades away. The only answer to this is to practise thinking the new ideas, so that they become ‘custom and habit’, and we build the strength of our ‘thinking muscles’.