McLuhan’s Tetrad of Media Effects 
McLuhan’s Tetrad of Media Effects focuses on 4 aspects or effects of media innovation on society:
- retrieval: “The recovery of values and insight that was once lost or eroded”
- enhancement:”A solution to previous problem.”
- reversal: “The unexpected dissatisfactions caused by the creation of a new problem.”
- obsolescence: “The erosion of formerly significant artifacts.”
The relationship of tetradic elements
Retrieval is to enhancement as obsolescence is to reversal.
What is retrieved is an outgrowth of the enhancement (an ancient version of the same metaphor). What is obsolesced creates opportunity for reversal.
Retrieval is to obsolescence as enhancement is to reversal
What is brought back, must also render something obsolete; what is enlarged will always do so at the expense of others.