Zevachim 80 - Do Not Add to the Torah and Do Not Subtract From It - How To Understand This?

If blood requiring four applications (such as that of a sin-offering) mixed with blood requiring one application (such as that of a first-born), then Rabbi Eliezer says that the mixed bloods should be applied with four applications, but Rabbi Yehoshua says that they should be applied with one application.

Said Rabbi Eliezer to Rabbi Yehoshua, "But if a Kohen were to perform only one application with this mixture, he would violate the prohibition not to subtract from the Torah!" Answered Rabbi Yehoshua, "And if he were to apply it with four applications, he would transgress the prohibition not to add to the Torah!"

Said Rabbi Eliezer in his defense "The prohibition not to add applies only when the blood is by itself, and here we have a mixture of bloods," to which Rabbi Yehoshua countered, "The prohibition not to subtract applies only where the blood requiring four-sided applications is by itself, but here it is mixed with blood requiring one application." And then Rabbi Yehoshua added a winning argument: "With your four application you actively transgress 'do not add' with your hands, whereas with my one application I only refrain from performing additional applications but I do not transgress with my hands."

Art: El Greco (Domenikos Theotokopoulos) - Nobleman with his Hand on his Chest -1583-85