A simple experiment, so simple as to seem trivial, will bring us directly to the heart of the matter. Take any two unequal objects, such as a pen and pencil or two unequally filled glasses of water, and place them on the desk in front of you. Then, partly closing your eyes to increase your attention to the task, pick up each one with the thumb and forefinger and judge which is heavier. Now introspect on everything you are doing. You will find yourself conscious of the feel of the objects against the skin of your fingers, conscious of the slight downward pressure as you feel the weight of each, conscious of any protuberances on the sides of the objects, and so forth. And now the actual judging of which is heavier. Where is that? Lo! the very act of judgment that one object is heavier than the other is not conscious. It is somehow just given to you by your nervous system. If we call that process of judgment thinking, we are finding that such thinking is not conscious at all. A simple experiment, yes, but extremely important. It demolishes at once the entire tradition that such thought processes are the structure of the conscious mind.
This type of experiment came to be studied extensively back at the beginning of this century in what came to be known as the Wurzburg School. It all began with a study by Karl Marbe in 1901, which was very similar to the above, except that small weights were used. The subject was asked to lift two weights in front of him, and place the one that was heavier in front of the experimenter, who was facing him. And it came as a startling discovery both to the experimenter himself and to his highly trained subjects, all of them introspective psychologists, that the process of judgment itself was never conscious.
Jaynes, Julian. The Origin of Consciousness in the Breakdown of the Bicameral Mind