When a subject learns about the world and gets experience as a result, he builds his own mental representations - thereby essentially differentiating the world (that is, dividing it into abstract categories). What can a person not differentiate?
Your question can be considered on different levels:
- Personal: The blind spots are what a person cannot see, and therefore is unable to discriminate.
- At presence: At each time there are general open questions. Examples of open questions in today’s science: Dark energy, the origin of the universe, how to reconciliate objective determinism and subjective free will.
- In general: Similarly like a dog cannot understand calculus or the problem of free will, I expect also the human species to have their mental limits. But it’s in the nature of things that we do not know these limits – like every person does not know its own blind spots.
In the 19th century the scientist Dubois-Reymond gave a speech: The Limits of Science. He coined the difference between questions of type "ignoramus, i.e. currently we do not know" and of type "ignorabimus, i.e. we will never know". Dubois-Reymond named several examples.
Your question is self-defeating. If a person can conceive the idea, then they can conceive the language to represent it; if they cannot conceive the idea, then they cannot conceive the language to represent it. Therefore, your question is really:
Use conceived language to describe that which cannot be conceived.
This must necessarily fail. Therefore, your question is essentially in the form of a koan. From WP:
The popular western understanding sees kōan as referring to an unanswerable question or a meaningless or absurd statement. However, in Zen practice, a kōan is not meaningless, and not a riddle or a puzzle. Teachers do expect students to present an appropriate response when asked about a kōan.
When one crafts categories, one is using cognitive processes to attribute properties to things, and then listing things as members of sets or types. In the classical method, this is done by definitions of necessity and sufficiency. As long as you have the ability to list a set of properties, you can create a category, even if they don't physically exist. Unicorns, kelpies, and square circles all have properties, for instance.
But you have created a category of things called things that have no properties and cannot be differentiated. This is a contradiction because we decide what things are according to their properties. Therefore, 'undifferentiated thing' is a fiction in the same way a square circle is.