Can somebody elucidate the distinction between Universality and Necessity? It seems to me that universality would dictate necessity, but I'm inclined to believe that, despite how I feel, this is not the case. Can someone provide me with an example of something that is universal but not necessary?
I think of it this way:
If x is universal, then we always observe x even though we can imagine and/or conceive of not observing it.
If y is necessary, the it is impossible to imagine and/or conceive of not-y, whether we observe y only once or thousands of times.
We can discover that some fact about nature is always and everywhere observed, and yet think without contradiction of nature being different. The specific rate of gravitation is an example of this, or the specific boiling temperature of water (taking into account variables like air resistance and air pressure); even though it's possible to conceive of the rate of gravitation being faster or slower than it is, or water boiling at a higher or lower temperature, we find as a matter of fact that these values are universal, constant.
For other aspects of nature, we are conscious that they must be the case. Even if there is only one triangle in the world, that triangle's angles will add up to 180 degrees; even if we only add 7 + 5 once and never engage in mathematics again, we are conscious that the sum could only be 12. The impossibility of conceiving/imagining/experiencing some other result is the mark of necessity.