Although insights may be somewhat unpredictable, they are usually born from careful reading and thought. If you are diligently seeking new sources and discovering new knowledge, insight will come.
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By now, you've probably read and evaluated many resources. Ideas may be floating around in your head, but you may not have a clear idea of your thesis yet. You may start to panic. "This is all great, but what am I actually going to WRITE about?," you may ask yourself. Don't fear. Nearly everyone experiences this uncertainty during the investigation phase of the process.
At some point, as you read and evaluate and think about your research question, an insight will come to you. For many people, this comes as a sort of "aha" moment. Suddenly you have a strong feeling about how you want to approach your paper. Pay attention to these moments. They happen when you've really begun to understand your research question and thought about it in a deep and authentic way. Now you're ready for your thesis statement.
If an insight hasn't happened yet, be patient, keep reading, questioning and thinking. An insight will come.
Critical thinking development: A stage theory - This article comes from The Foundation for Critical Thinking. It articulates the phases that people go through when they learn to think critically. Having an insight that leads to a productive thesis is an activity that engages your critical thinking skills.
Insights on the "aha" moment - This article from PsychCentral describes what's happening in the brain when an insight occurs.