derived unit, for example, the joule, the unit of energy. Its definition in terms of the base units is ⋅/. Even if the practical realisations of the metre, kilogram, and second are available, a practical realisation of the joule would require some sort of reference to the underlying physical definition of work or energy—some actual physical procedure for realising the energy in the amount of one joule such that it can be compared to other instances of energy (such as the energy content of gasoline put into a car or of electricity delivered to a household). The situation with the defining constants and all of the SI units is analogous. In fact, purely mathematically speaking, the SI units are defined as if we declared that it is the defining constant's units that are now the base units, with all other SI units being derived units. To make this clearer, first note that each defining constant can be taken as determining the magnitude of that defining constant's unit of measurement; for example, the definition of defines the unit as = ('the speed of one metre per second is equal to one th of the speed of light'). In this way, the defining constants directly define the following seven units: Further, one can show, using dimensional analysis, that every coherent SI unit (whether base or derived) can be written as a unique product of powers of the units of the SI defining constants (in complete analogy to the fact that every coherent derived SI unit can be written as a unique product of powers of the base SI units). For example, the kilogram can be written as Thus, the kilogram is defined in terms of the three defining constants , , and because, on the one hand, these three defining constants respectively define the units , , and , while, on the other hand, the kilogram can be written in terms of these three units, namely, While the question of how to actually realise the kilogram in practice would, at this point, still be open, that is not really different from the fact that the question of how to actually realise the joule in practice is still in principle open even once one has achieved the practical realisations of the metre, kilogram, and second. Specifying fundamental constants vs. other methods of definition The current way of defining the SI is the result