It naturally follows from the definition of the Continuous form (it denotes an action in a state of the process at the present moment or at a definite moment in the past or future) that verbs that do not express a process are not used in the continuous form.

The following groups of verbs do not express a process:

  • verbs denoting sense perception (to see, to hear);
  • verbs denoting mental activity (to know, to believe);
  • verbs denoting wish (to want, to wish);
  • verbs denoting feeling (to love, to hate, to like);
  • verbs denoting abstract relations (to have, to consist, to depend, to belong).

Overview

To See

In such expressions as to see the sights of, to see somebody home, to see somebody off the verb to see does not mean ‘to perceive by eyes’, so it can be used in the continuous

They were seeing the sights of London while their cousin waited for them at the hotel. (Wells)

The verb to see can be used in the continuous form to describe an ongoing action of observing or perceiving something over a period of time. For example:

I am seeing a lot of interesting things on my trip. (The speaker is in the process of observing or perceiving interesting things on their trip.)
She was seeing the sights of the city when I met her. (The speaker was in the process of observing the sights of the city at a specific time in the past when the speaker met her.)

The continuous form of to see is often used to describe an action that is happening at the present moment or was happening at a specific time in the past. It is not typically used to describe a future event or action, as the simple form of to see is usually used for that purpose.

Here are some more examples of sentences using the continuous form of to see:

We are seeing a lot of progress in our project. (The speaker and a group of people are in the process of observing progress in their project.)
He was seeing a therapist to help with his anxiety. (The speaker was in the process of observing a therapist at a specific time in the past to help with their anxiety.)

To Have

It is naturally possible to use the Continuous form of the verb to have in the expressions of the type to have dinner (lunch, supper) because it does not denote possession.

They are having lunch.

To Think

The verb to think cannot be used in the Continuous form if it denotes an opinion; it can if it denotes a process of thought.

I think you are right.

I am thinking of what you have just said.