We can create a system of objective measurement in a subjective reality. Though many thinkers believe that because we live in Plato’s Cave, and that the shadows projected on the wall are only observed through our interpretation, that does not mean we cannot create a system of repeatable mechanisms of measurement in order to align ourselves more objectively with reality. Take any form of distance measuring. Let’s take a meter, which is widely regarded as an accurate way to measure certain lengths and distances. Why is a meter the length that it is? Does it matter? What if we added additional length? Does adding or subtracting length, so long as afterwards we keep that version of the meter as the version of the meter to be used? It is only when the actual description of the length of the meter, which has been set to a certain amount of distance that light travels in a certain number of fractions of a second, then the meter accurately describes reality in that context.
This is not only just useful in measuring the physical world. We can use this principle to accurately measure other, less physical issues, so long as w continue to use a repeatable standard. Ethics, for example, as long as their principles remain constant, can be objective, despite there not existing a physical source from which ethics flows. This is not to say that all ethics are held equally; In fact that is to demonstrate that ethics that don’t meet their own standards should not even be held in the first place. The only reality in which truly subjective ethics, ethics that are meant be be twisted in order to achieve a goal, can thrive, is when those who are using a certain means to achieve an end intentionally in order to achieve the ends.