I think you’d have to call him a Modernist and a Postmodernist. As a Modernist, Pinter’s work is indebted to a naturalist/Realist tradition in that his dialogues are often so close to every day speech that they, ironically, tend to be ambiguous or downright absurd. Naturalism is an objective look at reality, like a botanist looking at the world from afar and realism is the approach of staying as close to reality as possible. What I mean is that he would include awkward pauses, sentence fragments, trailing off . . . and all the idiosyncratic ways we speak. This kind of ‘tape-recorded’ drama showed how our speaking is largely sparse, interrupted and occasionally nonsensical. So, in that respect, it is a prime example of Realism in that it capture reality as a mirror or a tape recorder would. Realism and naturalism were at their height at the beginning of the Modernist period (mid 19th-early 20th). But Modernism also had themes of uncertainty and wandering in a fast-changing world. So, the presentation of actual speech as stilted and confusing was a mirror depiction but it was also a metaphor of the growing alienation and confusion in a world that was becoming populated with faster machines (cars, planes) and larger, clustered cities often depicted as dark and alienating. Pinter’s work showed that his surface depiction revealed a deeper psychological or philosophical take on current/recent understanding of this period in history. The period being mid to late 20th century, which is the transition of Modernism to Postmodernism. Both movements shared some themes such as alienation, uncertainty, and particularly with authors like Pinter, Beckett and Kafka, the element of Absurdity. But Pinter’s experimentation with language is primarily Modern but has elements of the Postmodern in form and function.