We characterize the access to empirical objects in biology from a theoretical and philosophical perspective. Unlike physical objects, biological objects are the result of a history and their variations continue to generate a history. This property is the starting point of our notion of measurement. We argue that biological measurement is relative to a natural history which is shared by the different objects subjected to the measurement and is more or less constrained by biologists. We call symmetrization the theoretical and often concrete operation that leads to consider biological objects as equivalent in a measurement. Last, we use our notion of measurement to analyze research strategies. Some strategies aim to bring biology closer to physics, by studying objects as similar as possible, while others build on biological diversification.