In the simplest terms, the following describes the relationship between ontology, epistemology, and research paradigm.

Ontology examines the nature of reality
Epistemology examines how to examine reality

Ontology is interested in addressing the question “What is reality?”.
There are several ontologies that exist.

As an example three ontologies:

• the belief that there is only one single reality or truth.
• the belief that there are multiple realities.
• reality is constantly negotiated, debated, or interpreted

Epistemology addresses the question “How can I know reality?”

Epistemology refers to how an individual understands knowledge,
how they understand their own thinking process, and how they think others know.

There are also several different epistemologies that exist.

As an example three epistemologies:

• the belief that knowledge can be measured using reliable designs and tools.
• the belief that reality needs to be interpreted to discover the underlying meaning.
• knowledge should be examined using whatever tools are best suited to solve the problem.

When you combine your ontology and epistemology together, you are able to get a holistic view of how you understand knowledge.

Using the three ontologies and three epistemologies they can be combined to form the three most common research paradigms in social sciences:

• positivism
• constructivism
• pragmatism

Positivists have the ontology that there is one single reality or truth and the epistemological stance that knowledge can be measured.

In contrast, constuctivists believe that there are multiple realities and the epistemological stance that knowledge needs to be interpreted to discover underlying meaning.

Finally, pragmatists believe that reality is constantly negotiated, debated or interpreted, and the epistemological stance that knowledge should be examined using whatever tools are best suited to solve the problem.

Once you have determined your research paradigm, you are able to establish the approach and methodology that is best suited to examine the knowledge you are interested in.

These are notes on this video, credits go to their makers:

https://www.youtube.com/watch?v=hkcqGU7l_zU

But why, and this is my question, are all these approaches be separate?

These aproaches are in no means contradicory – Positivism or Constructivism or whatever..

They are stacked into each other like Russian dolls and therefor all equally true, or relevant and not contradictory but adding to each other. Aren’t they? This is one of these things that seem so obvious to me that I don’t understand how this is not the start of the story.

Simply because this is all within the world. It is the same thing, just different angles how to look at it.