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Think about things outside your normal range of thought, like the future of civilization and what creates meaning in life. Be the source of your own investigation. Since you're always available to yourself, any line of investigation about yourself (and there can be many) allows you to always make some progress. Consider the basis for what you believe. Why do you believe what you believe? Start from scratch and identify your reasoning.

Write down what you think about these subjects, including ideas you think you shouldn't write down (possibly because you think others may think they are stupid). While you may not be arriving at any striking conclusions, you will be exposing your own assumptions to yourself. You will probably marvel at how silly some of your assumptions can be, and in the process you will mature.