What, then, is value in exchange? It is the ratio in which one kind of useful commodity exchanges against another kind of useful commodity. This ratio, says Marx, does not in the least depend on the usefulness of the respective commodities, or their capacity of gratifying any particular want. For, first, that is a matter of quality, whereas value is a ratio between quantities; and second, two different kinds of utility cannot be compared, for they have no common measure; but value, being a ratio, implies comparison, and comparison implies a common measure. A fiddle charms the musical taste, a loaf satisfies hunger, but who can calculate how much musical gratification is equivalent to so much satisfaction of hunger. The loaf and the fiddle may be compared in value, but not by means of their several uses. Third, there are many commodities which are useful and yet have no value in exchange: air, for example, water, and, he adds, virgin soil. In seeking what in the exchange the value depends on, we must therefore leave the utility of the commodities exchanged entirely out of account; and if we do so, there is only one other attribute they all possess in common, and it must be on that attribute that their value rests. That attribute is that they are all products of labour. While we looked to the utility of commodities, they were infinite in their variety, but now they are all reduced to one sober characteristic they are so many different quantities of the same material, labour. Diversity vanishes; there are no longer tables and chairs and houses, there is only this much and that much and the next amount of preserved human labour. And this labour itself is not discriminated. It is not joiner work, mason work, or weaver work; it is merely human labour in the abstract, incorporated, absorbed, congealed in exchangeable commodities. In an exchange commodities are quantities of labour jelly, and they exchange in the ratio of the amount of labour they have taken in. Value, then, is quantity of abstract labour, and now what is quantity of labour? How is it to be ascertained? Labour is the exertion or use of man's natural powers of labour, and the quantity of labour is measured by the duration of the exertion. Quantity of labour is thus reduced to time of labour, and is measured by hours and days and weeks. Marx accordingly defines value to be an immanent relation of a commodity to time of labour, and the secret of exchange is that "a day's labour of given length always turns out a product of the same value." Value is thus something inherent in commodities before they are brought to market, and is independent of the circumstances of the market.