Well, I will start from the first third in the scale of C major, which will be C and E. Next come D and F, and in attempting to pass rapidly from the first third to the second one a difficulty will be immediately encountered. This is the ungovernable tendency of each finger to run apart from each other, and refuse to pull together at all. A purely mental difficulty though is this, and it can be overcome by training the mind, and accustoming it to govern the hands and fingers in complete independence one of the other. In fact I am convinced that in general, technical facility and control can only be obtained by great mental concentration, and not merely through mechanical practice.
That is why some people are able to learn to play a scale in thirds in an hour quite decently, because they possess the necessary power of brain, while others who may have quite as much musical talent will never master one at. all though they work six hours a day at it! I do not mean to say by this that thirds do not require an enormous amount of study, because of course they do, only to be successful the practice must be accompanied by much concentrated brain effort. Therefore one of the principal efforts of a good pianoforte teacher should be to stimulate in every possible manner the mental faculties of his pupils.