1. Preparation. The person expecting to gain new insights must know his field of study and be well prepared. This seems to fit what we have experienced 0 people get inventive ideas mainly in their own fields - poets in poetry; scientists, in science.
2. Incubation - Wallas noticed many great ideas came only a period of time spent away from the problem. This was certainly the experience of Archimedes when he got his idea in the public bath. Many ideas come to us when we are away from the problem, usually after actively engaging with the problem.
3. Illumination. The "click" or "flash" of a new idea. It's a mysterious phase. Resting the mind by doing other activities was the only suggestion Wallas could offer about how creative ideas form.
4. Verification. In this final step, efforts are made to see if the "happy idea" actually solves the problem. Since "great" ideas don't always work out in actual practive, this final step is vitally important to the success of any project.
We know that invention comes only in a person's field of specialisation. Wallas is right when he says there must first be a Preparation stage: people have to become knowledgable in some field before they may expect ideas to "dawn" on them in that area. (Probably the more we know, the more apt we are to get new ideas; novel ideas seem to come from a fortunate scrambling of information we already have). ANd yet, although a certain threshold level of knowledge seems necessary for creativity, creative breakthroughs are not always the product of the most expert thinkers in a discipline.
Roger von Oech presents a model of the creative process based on the Wallas model.
On to Osborne-Parne's CPS model.