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This work deals with the semantics of the French additive aussi. We make the broad hypothesis that its semantics closely match those of its English equivalent too. Thus, we rely on the usual formal analysis given to too, but will use experimental data on aussi. A complete analysis would test both items to check for differences, although as far as our knowledge is concerned, the differences are slim. Traditional analyses of too (e.g. (Krifka,1999)) describe it as a focus-sensitive and presupposition-triggering operator. For example, in (1), too associates with Mary and conveys the presupposition that someone different from Mary came (in that case: John). (1) JOHN left and MARY did #(too). The presupposition of too is anaphoric (cf. (van der Sandt and Geurts, 2001)): its antecedent must be salient in the discourse and cannot be accommodated. This explains why (2) is infelicitous, even though it is clear that Lemmy was not alone at dinner in New York. (2) # Lemmy had dinner in New York too. Apart from its presupposition, too is said to be semantically void, i.e. it does not contribute anything to the asserted part of its host utterance. Thus, in (1), the asserted part of the second conjunct is “Mary came”. Furthermore, as indicated in (1), too appears to be obligatory when its felicity conditions are met. This led a number of authors (see (Amsili and Beyssade,2009) and references therein) to treat this phenomena as a case of obligatory presupposition. We argue that the presence of a semantically compatible antecedent for the presupposition is not a sufficient condition for using too: its acceptability depends upon the segments being similar in the discourse (§1), a proposal we couch in an argumentative approach (§2). This proposal is then articulated with accounts of the obligatoriness of too (§3). §1: Some examples seem to contradict the description of too given above. For example, (3) is felicitous, but (4) is not, even though both have an antecedent compatible with too’s presupposition (or its negative counterpart either). (3) Lemmy solved only some of the problems and Ritchie didn’t solve all of them either. (4) # Lemmy solved only some of the problems and Ritchie solved some of them too. In (3), the antecedent is the asserted part due to only, i.e. “Lemmy did not solve more than some of the problems” (which entails that he did not answer all of them), whereas in (4), the antecedent is the presupposed part of only: “Lemmy solved some of the problems”. (5) shows that a presupposed proposition can serve as an antecedent for too’s own presupposition (as for most anaphoric items), and therefore that the presupposed nature of the antecedent cannot be the reason for the infelicity of (4). (5) Lemmy’s proud to be a bass player. Roberto plays bass too. Given that (4) becomes better without too (at least as an answer to the question “How many problems did Lemmy and Ritchie solve each?”), we conclude that its presence is responsible for the infelicity of (4), a fact unaccounted for in the traditional analyses of its meaning. Another set of problematic data is given in (6). There, each conjunct includes a modal along the scale 〈certain, highly likely, likely, possible〉. An element on the scale logically entails all elements below it, and is understood as being more probable than them. (6) A: Bordeaux and Marseille will each play a football match abroad. What are the odds? a. B: The victory of Bordeaux is certain, and that of Marseille is highly likely too. b. B: ? The victory of Bordeaux is certain, and that of Marseille is likely too. c. B: # The victory of Bordeaux is certain, and that of Marseille is possible too.