Cognitive biases can lead to misinterpretations of human and non-human biology and behavior. The concept of the
Umwelt
describes phylogenetic contrasts in the sensory realms of different species and has important implications for evolutionary studies of cognition (including biases) and social behavior. It has recently been suggested that the microbiome (the diverse network of microorganisms in a given environment, including those within a host organism such as humans) has an influential role in host behavior and health. In this paper, we discuss the host’s microbiome in relation to cognitive biases and the concept of the
Umwelt
. Failing to consider the role of host–microbiome (collectively termed a “
holobiont”
) interactions in a given behavior, may underpin a potentially important cognitive bias – which we refer to as the
Holobiont Blindspot
. We also suggest that microbially mediated behavioral responses could augment our understanding of the
Umwelt
. For example, the potential role of the microbiome in perception and action could be an important component of the system that gives rise to the
Umwelt
. We also discuss whether microbial symbionts could be considered in System 1 thinking – that is, decisions driven by perception, intuition and associative memory. Recognizing
Holobiont Blindspots
and considering the microbiome as a key factor in the
Umwelt
and System 1 thinking has the potential to advance studies of cognition. Furthermore, investigating
Holobiont Blindspots
could have important implications for our understanding of social behaviors and mental health. Indeed, the way we think about
how
we think may need to be revisited.