Psychology was first started as a scientific enterprise by Wundt in the late 1800's. The first psychologists examined their conscious thinking, then reported their observations. So they are called introspectionists.
Discovering Imageless Thoughts
It was natural for the introspectionists to list the types of thoughts they found. One type was sensory images. Sensory images include visual images, sounds, smells, etc., including the sight or sound of a word. A second type was intentions (such as your intention to read this).
In a study by Mayer and Orth, someone would say a word, then the subject would report the thoughts that occurred in response to that word. Mayer and Orth discovered that sometimes subjects had a thought that was not a sensory image or an intention. These thoughts came to be called imageless thoughts. They are essentially the same thing that Gendlin called "felt senses" and which are called "complicated thoughts" on this website.
For example, one subject reported a peculiar conscious experience that was not further definable. This illustrated how reports of imageless thoughts can be pretty sketchy. But this report exactly fits Gendlin's descriptions of felt senses -- there is a conscious experience, but the meaning of the conscious experience is not known.
In another instance, the stimulus was "mustard" and the subject's response was "grain.” The intervening thought was a conscious experience characterized as the memory of an idiomatic expression. So this imageless thought had some conscious meaning. But the subject didn't know what the idiomatic expression was or how it led from mustard to grain.
Finding Them Everywhere
Once the introspectionists discovered imageless thoughts, they found them everywhere. Gendlin had the same experience.
The introspectionists also found imageless thoughts in concepts. Consider the concept of cow. When you think 'cow', you probably construct the word 'cow' and a visual image of a cow. If you work at it, perhaps you can imagine the sound or smell of a cow. These are images.
But there is much more to your concept of cow than just these images. Some introspectionists claimed that the concept was an imageless thought.
Gendlin doesn't mention concepts being felt senses. But one of the focuser in his book does: "At first, when I tried to focus, I could never get a felt sense. All I had were words that I could feel, but there never was any feeling except right in the words. My words were like definitions and I had my feelings defined so they seemed as if they were exactly the same as the words. I was only looking at the center of each feeling, and in the center the feeling was what the words said. It took me three months till one day I noticed that there was more to the feeling. It had, sort of, fuzzy edges. They were beyond what the words got. That was the breakthrough for me. The feeling as having fuzzy edges, that's the felt sense."
Wundt, the founder of introspectionism, wrote: "The thought is present as a whole in one's consciousness before the first word is uttered. At the outset the focus of consciousness does not contain a single one of the verbal and other images which make their appearance in running through the thought and giving it linguistic expression." (Woodworth, 1938, pp. 784-785) And when you forget what you were going to say, you don't forget some of the words. Instead, you forget that single thought that you were going to express in words.
Arguing about Imageless Thoughts
There is a sad/ironic ending to this story. Science thrives on disagreement, but it can't tolerate disagreement about basic data, because there is no way to settle those disagreements. The introspectionist's basic data was the report of their conscious experience. When they disagreed – and they did – they could not settle their arguments. So they were doomed to failure. If one thing didn't sink them, the next would. But the actual trigger for their failure was imageless thoughts.
The introspectionists argued about whether any thought could be purely imageless. Some said yes. Some said no, every thought always produced at least some sensory image. This is probably a non-question. The key issue is that there there is some aspect of mental experience that does not contain images. The introspectionists agreed that this was true (Angell, 1911).
So, though it was doomed to failure anyway, introspectionism's direct cause of death was over a question that wasn't important. Psychology shifted. The basic data become the observable behavior and actions of their subjects. Psychologists could agree on this, and psychology was finally on a firm footing as a science.
However, conscious thoughts, instead of being the easiest thing to study, became the hardest. And certainly no one was going to resurrect the kiss-of-death topic of imageless thoughts. So the idea was basically lost to psychology.