English socialist and social psychologist Graham Wallas proposed four stages of the creative process in his book The Art of Thought, published in 1926.

These stages are:

• Preparation
• Incubation
• Illumination
• Verification

The preparation is the feeding stage. Your brain is hungry for knowledge, so you got to feed it. At this stage, your brain is like to sponge, soaking in everything, storing it, and making subconscious connections.

During the incubation stage, your brain is still making connections. Forcing it to come up with a unique and special idea during this stage is asking for too much. Let it do its work. It knows there is all this good material it has stored in its files. It hasn’t indexed that material yet.

Illumination is the stage when your brain comes up with great ideas, connecting pieces you had been looking for, causing you to leap out of your chair and scream “EUREKA!” These “lightbulb moments” happen at all sorts of awkward places—in the shower, just before you fall asleep, on long walks alone, or on a solitary drive in the car.

In the final stage, called verification, your brain takes that beautiful, shining lump of clay and molds it into the perfect statue. It evaluates the idea, verifying that it is a realistic idea, and starts building the surrounding framework to bring it to life.

The Art of Thought is out of print, but the following excerpt from it beautifully explains that our brain can be in one or all of these four stages at a time. They are constantly overlapping each other as we’re exposed to new exploration and experiences.

In the daily stream of thought, these four different stages constantly overlap each other as we explore different problems. An economist reading a Blue Book, a physiologist watching an experiment, or a businessman going through his morning’s letters, may at the same time be “incubating” on a problem which he proposed to himself a few days ago, be accumulating knowledge in “preparation” for a second problem, and be “verifying” his conclusions on a third problem. Even in exploring the same problem, the mind may be unconsciously incubating on one aspect of it, while it is consciously employed in preparing for or verifying another aspect. And it must always be remembered that much very important thinking, done for instance, by a poet exploring his own memories, or by a man trying to see clearly his emotional relation to his country or his party, resembles musical composition in that the stages leading to success are not very easily fitted into a “problem and solution” scheme. Yet, even when success in thought means the creation of something felt to be beautiful and true rather than the solution of a prescribed problem, the four stages of Preparation, Incubation, Illumination, and the Verification of the final result can generally be distinguished from each other.”

Source: Brain Pickings