In other words, he was describing teaching as an art in which the teacher is attuned to the speicific needs of each child. Tact according to the dictionary means "adroitness and sensitivity in dealing with others or with difficult issues." In other words, art.
I have been thinking of the thrust of modern education in which the greatest part of the teaching responsibility is being gradually shifted from the teacher to the mechanized device. Arthur Clark had said that "Any teacher that can be replaced by a machine should be!" So does that mean that the teacher of the future will become more like the self-checkout help at the big box store? Or will the teacher persist in his or her traditional role of keeping students artfully engaged in learning?
Perhaps an answer will be found in Salomon's next principle of educational Sloyd, "to teach through the senses, especially touch and sight." Interestingly, tact and tactile are from the same Latin root tactus "touch, feeling, handling, sense of touch," from root of tangere "to touch" (see tangent). Meaning "sense of "discernment, diplomacy, etc." first recorded 1804, from a sense that developed in French cognate tact.
Salomon described Sloyd as
"a means through which the pupil derives great advantage from the constant use of his faculties of observation and perception, through both sight and touch. This he cannot do so well when he sits a passive and patient listener to the dogmatic instruction of the teacher."
We will get better schools when we untie teacher's hands, and thence hold them to the highest standards. That of expressing tact.
Make, fix and create...