What exactly is the main idea in Thomas Henry Huxley's essay "the method of scientific investigation"?
'The method of scientific investigation is nothing but the expression of the necessary mode of working of the human mind. It is simply the mode at which all phenomena are reasoned about, rendered precise and exact. There is no more difference, but there is just the same kind of difference, between the mental operations of a man of science and those of an ordinary person, as there is between the operations and methods of a baker or of a butcher weighing out his goods in common scales, and the operations of a chemist in performing a difficult and complex analysis by means of his balance and finely graduated weights. It is not that the action of the scales in the one case, and the balance in the other, differ in the principles of their construction or manner of working; but the beam of one is set on an infinitely finer axis than the other, and of course turns by the addition of a much smaller weight.
This introductory paragraph, quoted from Thomas Henry Huxley's essay contains the primary focus of his essay. All he wishes to state is that all humans have the capacity of higher order analytical thinking, consciously or unconsciously.
The only difference between men of science, philosophy and other specialist fields and their style of thought in comparison to us, 'ordinary mortals' is that they share a greater awareness of how they think, because of the nature of their professions. We indulge in the same analytical thought processes, but we are not constantly aware that we do. Huxley uses the simple example of a sour and green apple to illustrate the point. Furthermore, the only other difference is that professional 'thinkers' think on a much 'finer' scale. Their thinking is attuned to the requirements of their art or profession. The thought processes involved though are inherently the same for everyone.