noun. The state or character of being uniform; uniformity.

noun. the quality of lacking diversity or variation (even to the point of boredom)

"Though perhaps, when they came to the use of speech, some regarding the uniformness of what was perceived, might call it the SAME thing: others, especially regarding the diversity of persons who perceived, might choose the denomination of DIFFERENT things."

similarity  

consistence  consistency  homogeneity  homogeneousness