 On the issue of measurability, the Representational Theory takes a middle path between the liberal approach adopted by Stevens and the strict emphasis on concatenation operations espoused by Campbell. Like Campbell, RTM accepts that rules of quantification must be grounded in known empirical structures and should not be chosen arbitrarily to fit the data. However, RTM rejects the idea that additive scales are adequate only when concatenation operations are available (Luce and Suppes 2004: 15). Instead, RTM argues for the existence of fundamental measurement operations that do not involve concatenation. The central example of this type of operation is known as “additive conjoint measurement” (Luce and Tukey 1964; Krantz et al. 1971: 17–21 and Ch. 6–7). Here, measurements of two or more different types of attribute, such as the temperature and pressure of a gas, are obtained by observing their joint effect, such as the volume of the gas. Luce and Tukey showed that by establishing certain qualitative relations among volumes under variations of temperature and pressure, one can construct additive representations of temperature and pressure, without invoking any antecedent method of measuring volume. This sort of procedure is generalizable to any suitably related triplet of attributes, such as the loudness, intensity and frequency of pure tones, or the preference for a reward, it size and the delay in receiving it (Luce and Suppes 2004: 17). The discovery of additive conjoint measurement led the authors of RTM to divide fundamental measurement into two kinds: traditional measurement procedures based on concatenation operations, which they called “extensive measurement”, and conjoint or “nonextensive” fundamental measurement. Under this new conception of fundamentality, all the traditional physical attributes can be measured fundamentally, as well as many psychological attributes (Krantz et al. 1971: 502–3).