Note Previous Page Naturalistic Method
In this particular sense, Participant Observation is a naturalistic method because it involves the researcher:

"Telling it like it is" or, if you preferTo really understand what it's like to be a postal worker you may have to become one...

"Really understanding, through personal experience, what is going on in any given situation".

In other words, Participant Observation is a method that attempts to understand the motives and meanings of people's behaviour from the viewpoint of those involved in the behaviour being studied.