This part of the thesis would be considered the topic, more specifically it would be considered the <em>introduction of the topic to study</em> since the thesis revolves around, I assume, Isaac Newton and you are offering a brief 'summary' of his life and accomplishments.
However if the reason to mention Sir Isaac Newton is to support an idea or a statement this could be considered a subtopic. It is subjective and directly related to the field of study you're doing the thesis for.
