 Other early attempts at clarifying the concept of levels were carried out by individual researchers working alone on similar or related issues, such as reduction or multilevel selection. Mario Bunge (1960, 1977), Marjorie Grene (1969, 1987), and William Wimsatt (1976a, 1981, 1994) each developed several ground-laying observations concerning levels. Bunge (1960) remarks on the plethora of meanings the term can possess, then offers a long taxonomy of nine different meanings of the term “levels”, ranging from the innocuous (levels as degrees or quantity, as in ‘levels of stress’) to the sense of nature being ordered into an evolutionary series, with several corollary senses of levels falling between (such as degrees of complexity). In his 1977 paper, Bunge goes further, offering a set-theoretic treatment of levels, and claiming that the leveled structure of the natural world is what renders natural phenomena intelligible to us (Bunge 1977: R82). Grene (1969) also remarks on the polysemic nature of the term “hierarchy”, noting that ‘levels’ has distinct uses in different fields of sciences. Grene later (1987) distinguishes between two notions of hierarchy, one falling under a mixed notion of levels as composition and control, respectively, and the other exemplifying phylogenetic ranks. Wimsatt’s work will be considered in more detail below in section 2.3.