The human ability to think is tightly tied to the ability to use words, symbols, and images. The capacity to think is then related to the capacity in which we use instruments: words, symbols, images, concepts, categories, definitions, etc.
The human being thinks through concepts. A concept is a classification that helps to understand the world around. Man forms concept to help him think with speed and clarity. Concepts are made for communication and the success of that communication depends on the exactitude with which the concepts agree with each other. Wrongly defined words impede clear thinking.
When studying, consistent criteria should be used to discern the true from the false. We should abandon all ordinary justifications and hold all knowledge under the most rigorous test. It is this doubtful attitude that should be placed before every test and all knowledge which we should adopt and cultivate in our studies.
People ignore the evidence that contradicts their ideas, and the more vigorously they sustain their ideas, the more reluctant they are to accept the evidence. It is essential to use all evidence available when study and weigh them out to reach all conclusions and get the best of all of them.