What is measurement and what can it tell us about the quantity measured? Can we know a quantity by measuring it? We mathematically demonstrate that the answer is no! We show how a continuous quantity E(t) that grows exponentially can in our measurements of it be seen as discrete and growing linearly. And if we further consider the practical limitations that render measurements as 'approximations' only, then the quantity E(t) that we measure can be any integrable function yet our measurements of it will still depict it as discrete and linear. Furthermore, and most urprising, the 'interaction of measurement' will be described by Planck's Law, whether E(t) is exponential or just integrable. Thus, we cannot know what the hidden quantity E(t) is by the measurements of it.