Ever since Johan Huizinga's reassessment of the medieval mind (Herfstij der Middeleeuwen, 1919) as a passionate intensity, scholars have sought to understand pre-scientific perceptions of nature. García-Osuna's medieval Ethea captures this point; the hero's moral framework and perception strategies reflected their assimilation of society's communal codes for perceiving, judging, and acting: ‘Those codes are the source of the hero's supply of implicit knowledge’ (p. 222). At the centre of this study is a reconsideration of the role of myth in the medieval view of the Atlantic Ocean. García-Osuna proposes that an explanation of reality offered by myth ‘is just as valid as that offered by science in the sense that, as a human social construct, “reality” … is effectively intertwined with human experience. …The legitimacy of myth, then, flows from its ability to supply human beings with a coherent, qualitative method of interpreting reality through metaphorical patterns and symbolic archetypes’ (p.10). Myths are rooted in the community's collective consciousness and imagination that allows one to travel beyond any material world easily observed, for ‘without imagination, the superficial world of objects would be the measure of “all there is” … Myth is continual imagination’ (p. 15). The medieval traveller structured their narrative identity within ‘a sheltering infrastructure of clan and tradition’ (p. 197).

The Atlantic Ocean was a gateway to another world; it had immense power, even over the sun. Although the sun provided light and heat, it appeared to be extinguished in the ocean as it disappeared each evening, yet was born of the sea each morning. The ocean was a liminal space, whereby the traveller searched for a distant island where reality and time were suspended, and one could see magnificent things. In entering the ocean, ‘this other sphere is a mental space as much as it is physical, a hypothetical cosmos through which the medieval psyche drifts as in a dream, roaming free, unconstrained by the limitations of a map’ (p. 27).

Pre-Christian journeys were designated as echtrai, the old Irish word for adventure to the otherworld of Tir Tairngire, the Celtic paradise or land of promise. By the late seventh century the Christian journey became known as Immrama (or Immráma, literally rowing about) and in some cases it is motivated by the need for penance (Snedgus and Mac Ríagla) or vengeance (Máel Dúin). One of the well-known stories is Brendan's Navigatio, and this Immrama became the primary means by which Christian proselytisers legitimised their doctrines and beliefs; however, the pagan substratum in Immrama remained for centuries. The Irish otherworld was a material space and therefore Christian saints needed to become seafarers to gain credibility for their claims. When the Norsemen first arrived in Iceland they discovered Irish books, bells and crosiers. Thus, the immram journey proves that Christianity is based on universal and self-evident truths. In turn, the Irish church's concessions challenged the Roman church's official set of doctrinal beliefs. These concessions created a unique Irish Christianity that had a ‘syncretic character’, and in this way it paralleled older Irish mythology with basic Christian beliefs, so that many older practices (such as reverence for holy wells) appeared as a natural development (p. 31). García-Osuna reminds us that many continental theologians criticized the Irish church for this; however, one could also make the argument that the increased sale of relics on the continent pointed to its own syncretism; certainly, Erasmus made this point. Indeed, if the world was sea, the church would become the ship (p. 82).

García-Osuna poses an important question: if the bible contains all the knowledge that Christians need for salvation, what is the purpose of Immrama? In the case of Brendan's Navigatio, one can consider three points. First, ‘ostensibly, Brendan's interpretation of the biblical passage “And everyone who has left houses or brothers or sisters or father or mother or wife or children or fields for my sake will receive a hundred times as much and will inherit eternal life” (Matthew 19:29)’ (p. 204), was a powerful incentive to depart his own lands. Secondly, his voyage was also an exploration of ‘the periphery of the human self, an enigmatic expanse symbolised by the Atlantic Ocean’ (p. 204). This required the hero to return from his adventure to become a mentor to others with his new enlightened message. Thirdly, Brendan needed to conform to social expectations since the quest had to display all the expectations of its audience. He had to ‘accomplish a sort of thaumaturgy: to denature the story's pagan essence’ (p. 205). Thus, from the medieval perspective, the story's verisimilitude did not depend on how well it described a material reality, but its description of the expected mental landscape. ‘Free from the demands of plausibility’, medieval Christian Immrama accounts like Brendan's Navigatio had the power to transform the stories and characters of Irish mythology to help understand basic Christian beliefs: ‘Realistic portrayals would only serve to conceal what really counts for the writer and reader behind a complicating, vast volume of extraneous information’ (p. 206). García-Osuna has concluded that in contrast to the continental Christian doctrine of Augustine, where faith seeks understanding, for the medieval Irish Christian the formula would have been altered to ‘men in boats seeking understanding’ (p. 228).