Abstract relations are considered the pinnacle of human cognition, allowing for analogical and logical reasoning, and possibly setting humans apart from other animal species. Recent experimental evidence showed that infants are capable of representing the abstract relations same and different , prompting the question of the format of such representations. In a propositional language of thought, abstract relations would be represented in the form of discrete symbols. Is this format available to pre-lexical infants? We report six experiments ( N = 192) relying on pupillometry and investigating how preverbal 10- to 12-month-old infants represent the relation same . We found that infants’ ability to represent the relation same is impacted by the number of individual entities taking part in the relation. Infants could represent that four syllables were the same and generalized that relation to novel sequences (Experiments 1 and 4). However, they failed to generalize the relation same when it involved 5 or 6 syllables (Experiments 2–3), showing that infants’ representation of the relation same is constrained by the limits of working memory capacity. Infants also failed to form a representation equivalent to all the same , which could apply to a varying number of same syllables (Experiments 5–6). These results highlight important discontinuities along cognitive development. Contrary to adults, preverbal infants lack a discrete symbol for the relation same , and rather build a representation of the relation by assembling symbols for individual entities.