Let's say how we 'ought' to think. Thinking in terms of 'fundamental'. An example, a two story house. It has a foundation, roof, floor etc. Everything depends on everything. It has structure. Certain ideas are the ground floor or foundation of cognition, other parts are dependents to form the whole. Thinking in terms of fundamentals means never accepting a conclusion while ignoring its base, it means knowing and validating the deepest ideas on which one's conclusion rests.
Lying is wrong because it is incompatible with the requirements of self-preservation. 'Why should a man who is committed to selfless service necessarily tell the truth?' Most people want him to lie for their own happiness. Lying is self-destructive. A policy of lying leads to a war against reality, which no one can win. There is a reality, our minds are able to know these facts, to know reality. The issue of lying is merely a consequence, a derivative which rests on a complex philosophic foundation.
So it boils down to the issue of fundamentals which is a large part of 'thinking in principle. And our thinking, we are basically trying to understand the world without confusion, floating abstractions. This requires logic, logical structure, back to the fundamentals of philosophy.
Thinking about say Capitalism or Freedom. We know what they are but what do they depend on? How they can be proved? All the way back to metaphysics and epistemology. Everyone needs a system of thought, a complete philosophy of life.