When the number of incoming stimuli is reduced, the mind gradually perceives finer details in the ones that remain. This is famously true of blind people, but you can consciously narrow your attention to the same effect. The depth of detail revealed is likely to be surprising, since the generalist day-to-day mind imagines itself to be very capable.
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When you procrastinate by reading a dozen articles online, you might justify it by imagining how much you're learning by doing so. But that's a shallow kind of learning. It gives you knowledge that may be temporarily or tactically useful, but it can't give you new understanding.
Turning knowledge into understanding requires the work of sustained thought: examining the assumptions, internal structure, and consequences of the material. Without this work, you can have a reference to an idea, but it won't be fully available to your subconscious. With the work done, the idea gets fully copied into the mind, where by nature it interacts with other ideas and makes itself available when it would be useful.
Therefore, rather than reading a dozen things, you'd be better off spending the same amount of time on reading the one deepest thing, maybe more than once, and doing the work to absorb it.
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The mind takes time to sink into a task. It stands to reason that the fundamental principles that led to hierarchical memory in computers also apply in the brain, and there may be a similar hierarchy of some computational resource, like task availability or priority. It sure feels like it, and there is some supporting evidence for the notion.
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When proving theorems, learning to write creatively, or building a software application, you first pass through the part of work that others have done before. Only once you've gotten through that, to what's novel about your contribution, can you produce value commensurate to the effort you put in. (In fact, it probably becomes superlinear.) Someone who pursues more goals spends more of their time in the initial phase.