Not everything you say in your thesis needs to be supported by a reference. It’s OK to present your own thoughts and observations, and you don’t always have to refer to someone else who’s already said it.
If you want to describe a technique or a theory, try describing it in your own words based on your own understanding and experience.
So instead of saying, “according to Smith, this theory is useful because…”, have the courage to state your own reasons.
You should of course cite sources where appropriate, but you should also include thoughts of your own. The examiners want to know what you think about your subject, not just what you’ve read about it.
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