> In reality, our 5-volt CMOS system will consider anything above 1.67 volts to be a 1, and anything below 1.67 to be 0.
Worth noting that the region from 1.67 V to 3.33 V is undefined and systems in practice will not behave nicely for signals in this range. A CMOS logic 1 needs to be above 2/3 Vdd to be reliably recognized.
Any practical binary logic scheme is going to have an undefined zone in the middle because the gain of the devices used for the logic is not infinite and there would be problems caused if the gain was too high.
Worth noting that the region from 1.67 V to 3.33 V is undefined and systems in practice will not behave nicely for signals in this range. A CMOS logic 1 needs to be above 2/3 Vdd to be reliably recognized.