This is a discussion question in which argumentation can be practised.

Psychological reality is a difficult concept but one that needs discussion. Here it means that constituents as units are not just a formal way of describing the language but are used in production and comprehension by users. The evidence for these units comes from speech errors where we see that units can be substituted, anticipated, perseverated, or exchanged (Section 2.4.2). Consider, for instance, the following spontaneous English speech error:

I got into [this guy] with [a discussion] (intended: into a discussion with this guy)

Clearly, what is exchanged here are not nouns, but noun phrases (combinations of nouns with an article or demonstrative pronoun); the example thus shows that NPs have psychological reality. Similar errors could occur in a sign language.