• ontology - the metaphysical study of the nature of being and existence
• metaphysics - the philosophical study of being and knowing
• epistemology - the philosophical theory of knowledge
• a priori - derived by logic without observed facts
• a posteriori - derived from observed facts
• rationalism - the philosophical doctrine that knowledge is acquired by reason without resorting to experience
• empiricism - the philosophical doctrine that knowledge derives from experience
• relativism - the philosophical doctrine that all criteria of judgement are relative to the individuals and situations involved

From Wikipedia:

Ontological perspectives

Social scientists adopt a number of approaches to ontology. Some of these are:

• Realism - the idea that facts are "out there" just waiting to be discovered;
• Empiricism - the idea that we can observe the world and evaluate those observations in relation to facts;
• Positivism - which focuses on the observations themselves, attending more to claims about facts than to facts themselves;
• Grounded theory - which seeks to derive theories from facts;
• Engaged theory - which moves across different levels of interpretation, linking different empirical questions to ontological understandings;
• Postmodernism - which regards facts as fluid and elusive, and recommends focusing only on observational claims.

https://en.wikipedia.org/wiki/Ontology