But if you accept that you can trust your own thoughts, even if they aren't always true, and that what you see is real for your brains, even if that is not necessary the case, then you have a base on which you can build.

This is why the sentence: "I think therefore I exist " from Rene Descartes is seen as so important. But it is more correct to write, "I think and/or I feel therefore I exist". As to why the use of and/or this will be explained later in the text.

Skepticism is useful as it allows one to question his/her observations and thoughts. And to weed out the ones that are false. This might take time, but is very useful. If there would exist a course called "Learning to think" than skepticism should be a part of it. An other thing that would need to be included is the existence of bias, and subjectivity. We often seem to think we are not biased in our opinions, but to be really objective is not so easy.

If one accepts that the mind that produces thoughts is the result of a finite amount of electronic processes, in the human brains, then take this one step further, and accept data from neurological research, the empiric data. In the end one comes to a point where there are many possibilities in the difference between real and semblance is part of that and can be observed. And skepticism can not always figure those out.

Somebody whose arm has been amputated just above the elbow or just below, can still feel the itching feeling of a piece of arm he/she no longer has. This is because the nerve cells that had to do with that piece of arm still exist in the brain. This for a part shows that sensations and feelings can be observed to be real by the person while that what they point to no longer exists. (source http://www.newscientist.com/ )

This of course, poses a question : what can be seen as real and what not ? It has already been indicated that the possibility could exist that there are women [and perhaps also xxy's and variants] who have more than the usual types of receptors for the colour green, and as such who can see more different colours of green than other people. (source http://www.newscientist.com/).

At the other hand the people who see colors with certain words, are not mad. Apparently one is born with that ability and you cannot learn it. [Source: http://www.newscientist.com/] If one would put these two groups of people together with people who do not have either or both abilities, then the possibility that those last people will say that the first are not telling the truth, or that they are misinterpreting things, imagining things, is very real.