After the success of QM. They want to extend it to fields like electric field, electromagnetic field.. hence the initial attempt at quantum field theory (QED).. next they think particles like electrons can be treated the same.. hence the so called second quantization. Besides fields.. what else are there that must be quantized (or quantumized)? They tried to quantize geometry but can't seem to be successful. What is the criteria before things can be quantized.. like why do they not quantize economics or others? Why only fields? Nothing else? Is reality this simple?