My latest philosophical studies revolved around natural philosophy and particularly time. Over the course of my study I found the Scholastic formulation that time is fundamentally extra-mental, but formally and actually mental. This goes for space as well. Time is the measure of motion according to before and after. Motion extra-mentally exists only in the moment. It does not have an actual extended existence. However, time is extended. Time may be called abstract motion and the Angelic Doctor speaks of time existing in things in only a limited way.
Now economic value has the similar ontological structure. It is fundamentally extra-mental, grounded in the actual powers of things. However, it is formally and actually mental. Time measures motion, but value measures usefulness to men. Value measures the magnitude of which the powers of a thing are useful to man in his ends. This is as the Philosopher says a measure of need or demand, but it is a measure of the thing itself as being able to meet those needs and demands.
The difference between time and value show something of the distinction between the theoretical and the practical sciences. Time measures motion, which provides theoretical knowledge about motion. Value measures a thing as useful to man, which provides of practical knowledge concerning of whether to obtain it and importance of its use to man. In this way, time remains to a great degree more objective than value even though they have the same ontological structure. Time pertains to our understanding of things as extra-mental while value pertains to things as useful to men and men in their actions and ends.
This I think explains the reason that in modern thinking time is reduced to the extra-mental while value is reduced to the mental. This is not a reduction in the sense of eliminating the foundations or the mental aspects, but in taking what is at one level and putting it into another. Hence, time is considered an absolute reality apart from human thought and the consequence is eternalism. Value is considered a purely subjective measure, which leads to the notion that human desire is itself the principle of value.