The question is raised as to which values to select over others. The combination of references to substitution also serves to make the point that making choices and choosing one thing over another is unavoidable.
This unavoidable nature relates to the fact that substitution is part of society. We also see where substitution is of benefit to a person. Elbow is one example, he owes his job to substitution, as he says "as they are chosen, they are glad to choose me for them." (II.i) This leads to one of the themes of the play, corruption, a theme that touches on substitution.
In fact there are many themes of the play that relate to substitution. Some of the themes are a choice between two things, where one must be substituted by the other. Action vs. words is one example. This theme is especially important as it also relates to the nature of appearances, in that what appears is not always actual. The Duke is one example, he announces that he will be leaving the city but sneaks back in disguise. This relates to another theme, appearance vs. reality. Angelo appears to be a good man, but we find that he is not. Disguise is another theme, where disguise can be seen as substituting the real person for another. The Duke takes on a disguise as the Friar. While this is an obvious disguise there are also the less obvious. Angelo is disguised as a good man, in the end he is shown not to be. It is significant that it is the Duke's disguise that is the downfall of Angelo's. This suggests that it is in the purpose of the disguise that the importance lies. Manipulation is also a key component in the play, manipulation is the reason for the events of the play. Without it Claudio would have died, Angelo would have remained unexposed, and Mariana would not have gotten married. Manipulation is in itself a substitution, the process of substituting what is real for that the manipulator wants you to believe.
In the play manipulation is not a bad thing, disguise is for the Duke but not for Angelo, reality is seen to triumph over appearances in the end and action is seen to triumph over words. All of these themes relate back to substitution, the difficulty is that the combination of many similar themes leaves the audience with no clear meaning as to the nature of substitution. Just as in the play and in life itself, the choice is not clear.
This is what the play is all about, the nature of choices. Deciding one thing means another is not chosen, so making a choice is a matter of substitution. In the play, the characters have difficult choices to make. Angelo decides the fate of Claudio, and condemns him to death. Isabella decides whether it is more important to save a life or save a soul. The play is also about the choice between mercy and justice. Isabella becomes the symbol of mercy when she pleads for Angelo, the man who tried to seduce her and who condemned her brother. In a similar fashion, the Duke also reveals his mercy when he pardons Claudio, Lucio, and Angelo. The Duke feels he hands out appropriate justice based on the nature of the crime, measure for measure. It is this that sums up the nature of the play, measure for measure. Each character is doled out what they gave out. In the end it is not a question of society, but a question of each character.
The same thing can be understood when we compare the Duke and Angelo. The Duke takes on a disguise and this disguise becomes the downfall of Angelo's disguise. The meaning here can be interpreted as showing that it is what is inside a man that is important and that nothing can disguise a person's character. The true meaning of the play may be that measure for measure, what you get is based on what you are and nothing can substitute for that.
Brown, Donohue, Axnick, Blount, Ewen, Jones. Syphilis and Other Venereal Diseases. Harvard University Press. Cambridge Massachusetts, 1970.
Rosebury, Theodor. Microbes and Morals. The Viking Press. New York, 1971.