I have had some issues understanding this topic. On two occasions Dale has pointed out to me that they are not really based on physical things, given that their values are completely arbitrarily defined for the purpose of matching certain measurable or defined things, like the ampere and the speed of light. What I've read about them is that their definitions are, respectfully, how the substance influences the magnetic flux in the region it occupies and how a substance/object holds energy in an electric field. What I don't understand is, assuming the definitions I've seen are remotely close to correct: aren't those things physical? Now, I get that you can use whatever units you want. You can set the speed of light to 1, for example. The meter, obviously is a defined quantity. As far as I can tell, as far as it goes for units for speed, acceleration, jerk and so forth, only the definition of the second has real physical origins, given that an atom is used to define it. But isn't the way an object reacts to magnetic or electric fields something physical, regardless of whatever units we make up for it? As far as I can tell, if they aren't really anything to do with reality, then why is the magnetic field or electric field? Are those too merely made up constructs that give us values that match measurements? This is clearly a big point of confusion for me. I'd appreciate any elaboration on this. Or resources with good explanations. Thanks to all!