Consider a simple circuit with just a battery connected to a resistor. Ohm's law states that the current flowing through the resistor will be proportional to the voltage across the resistor. This is verified practically by connecting a voltmeter across the resistor and an ammeter between the resistor and the battery, measuring the current and voltage and finding that their ratio remains constant.

But just for the sake of argument, is not the circuit with the meters different from the circuit without the meters? Couldn't the meters have "done something" to the circuit which made the current and voltage proportional? Unfortunately, it is not possible to measure the current and voltage in the circuit without attaching meters to the circuit and thus possible altering its nature. Therefore, can Ohm's law ever be experimentally verified (or proved for that matter)? I like to side on the answer, "No!" ('coz it's a lot more fun to think about it that way!)

You could argue that the resistance of a voltmeter is very large and that of an ammeter is very small and both would not considerably alter the overall resistance of the circuit. But then again the fallacy in such an argument is that concepts of series-parallel equivalent resistance are all derived from Ohm's Law!

Extending this idea, the state of a system cannot be measured or verified without modifying the system with some sort of measurement device. This addition of a measurement device could possibly alter its nature and thus not provide the "true" state of the system. An application of this generalization yields the amazing result that a luminous body such as a candle flame emits light only because there are eyes to see it! And sea waves striking the shore produce sound only because there are ears to hear it! And so on…