 Considerations of this sort, based on the distinction between extensional and intensional readings of the connectives, were not original to Lewis. Already in 1880 Hugh MacColl in the first of a series of eight papers on Symbolical Reasoning published in Mind claimed that \((p\rightarrow q)\) and \((\neg p\vee q)\) are not equivalent: \((\neg p\vee q)\) follows from \((p\rightarrow q)\), but not vice versa (MacColl 1880: 54). This is the case because MacColl interprets \(\vee\) as regular extensional disjunction, and \(\rightarrow\) as intensional implication, but then from the falsity of \(p\) or the truth of \(q\) it does not follow that \(p\) without \(q\) is logically impossible. In the second paper of the series, MacColl distinguishes between certainties, possibilities and variable statements, and introduces Greek letters as indices to classify propositions. So \(\alpha^{\varepsilon}\) expresses that \(\alpha\) is a certainty, \(\alpha^{\eta}\) that \(\alpha\) is an impossibility, and \(\alpha^{\theta}\) that \(\alpha\) is a variable, i.e., neither a certainty nor an impossibility (MacColl 1897: 496–7). Using this threefold classification of statements, MacColl proceeds to distinguish between causal and general implication. A causal implication holds between statements \(\alpha\) and \(\beta\) if whenever \(\alpha\) is true \(\beta\) is true, and \(\beta\) is not a certainty. A general implication holds between \(\alpha\) and \(\beta\) whenever \(\alpha\) and not\(-\beta\) is impossible, thus in particular whenever \(\alpha\) is an impossibility or \(\beta\) a certainty (1897: 498). The use of indices opened the door to the iteration of modalities, and the beginning of the third paper of the series (MacColl 1900: 75–6) is devoted to clarifying the meaning of statements with iterated indices, including \(\tau\) for truth and \(\iota\) for negation. So for example \(A^{\eta \iota \varepsilon}\) is read as “It is certain that it is false that A is impossible” (note that the indices are read from right to left). Interestingly, Bertrand Russell’s 1906 review of MacColl’s book Symbolic Logic and its Applications (1906) reveals that Russell did not understand the modal idea of the variability of a proposition, hence wrongly attributed to MacColl a confusion between sentences and propositions, as Russell conferred variability only to sentences whose meaning, hence truth value, was not fixed. Similarly, certainty and impossibility are for Russell material properties of propositional functions (true of everything or of nothing) and not modal properties of propositions. It might be said that MacColl’s work came too early and fell on deaf ears. In fact, Rescher reports on Russell’s declared difficulty in understanding MacColl’s symbolism and, more importantly, argues that Russell’s view of logic had a negative impact on the development of modal logic (“Bertrand Russell and Modal Logic” in Rescher 1974: 85–96). Despite MacColl’s earlier work, Lewis can be regarded as the father of the syntactic tradition, not only because of his greater influence on later logicians, but especially for his development of various systems containing the new intensional connectives.