이유태의 공식적인 예송참여는 기년설에 대한 남인계의 파상적 공세와 서인계의 예설 분열이 나타난 시점이었다. 이때 그는 공적으로 兩宋(송시열 송준길)의 입장을 지지하였고, 또한 삼년설로 설득하고자 한 윤휴와는 절교하는 대신 예설을 지어 서인계의 禮說 共助를 분명히 함으로써, 1차 예송에서 서인이 승리하는 데 일조하였다. 그러나 2차예송에서 서인이 패배한 후, 그는 송시열과의 예설공조를 거쳐 「갑인설」을 완성하였으나 후에 그것이 자신의 解配를 위해 改變된 것이라는 소문으로 송시열과 갈등하게 되었다. 그는 「병진설」 등을 지어 자신의 입장을 적극 해명하였으며, 후에 김광현은 초려신도비에서 이 갈등이 남인계의 誣達에 의한 것이라고 규정지었다.
이유태의 服制說은 요컨대 효종은 ‘본래의 適長子’는 아니고 ‘昇爲適, 즉 서자로서 올려서 된 적자’로서, 종통계승의 정당성은 인정되지만, ‘본래의 次子’로서의 종법적 위상은 여전함으로 조대비의 상복은 기년복이어야 한다는 것이다. 따라서 그의 왕조례 예설에 나타난 예사상은 네 가지로 요약된다. 첫째, 周公宗法과 성리학파의 宗法認識에 대한 존신이다. 주공종법은 은나라의 兄弟相續을 부정하고 嫡長子繼承을 절대시하는 친족조직의 원칙으로, 질서있고 안정된 종통계승을 목표로 제정된 것이며, 성리학파는 이 원칙에 대개 충실하였다. 둘째, 易地思之의 情禮認識이다. 이것은 그가 易地하여 예제의 적용대상자를 배려하는 입장을 말한 것이다. 셋째, 禮治主義的 예인식이다. 이것은 그가 反宗法主義的 예설에 대해 우환의식을 가지고 종법주의에 의한 禮治를 추구하였음을 뜻한다. 넷째, 天下同禮의 예인식이다. 이것은 왕조례와 사대부례의 종법운영이 하나의 잣대에 의해야 한다는 입장이다. 그것은 17세기 서인계 산림의 예치주의적 방법론의 제시였고, 당시인들의 보편지향적 시대의식을 선도하는 진일보한 것이었다.
Lee Yu-tae(李惟泰) began to officially participate in disputes over prescribed mourning in the fourth month of the first year of King Hyeonjong when Southerners〔南人系〕 launched a series of attacks on the theory supporting a one-year mourning period and Westerners〔西人系〕 began to be divided over theories on rites. At that time, he firmly supported the assertion of two Songs, Song Si-yeol(宋時烈) and Song Jun-gil(宋浚吉), in front of the King. In addition, he declared the severance of the relationship with Yun Hu who said "Persuade Song Si-yeol to support the theory supporting a one-year mourning period," explicitly showing that he agreed to Westerners' theories on rites, and contributing to Westerners' wining 1st dispute over a prescribed mourning period. After Westerners lost 2nd dispute over a prescribed mourning period, he completed 「Gabinseol」(甲寅說)based on his cooperation with Song Si-yeol over theories on rites. Later, he had conflicts with Song Si-yeol because of the rumor that the content of 「Gabinseol」 did not represent what he believed, and that he wrote the book only for the purpose of gaining his release from banishment, He, however, argued that his theory on rites was not different from that of Song Si-yeol, and later his Sindobi(神道碑), a sort of tombstone, says that such conflict was attributed to Southerners' fabricated rumor.
With regard to a matter of wearing mourning clothes, Lee Yu-tae argued that Jodaebi(趙大妃), queen of Ikjong was required to wear mourning clothes for a year for King Hyeojong, Jodaebi's second son elevated to the position of the eldest son, because Hyeojong's status by clan rules remained unchanged though he became the eldest by legitimacy after his elder brother's death. In the meantime, Lee Yu-tae's thoughts on ritual can be summarized into four parts based on theory on royal rites. First part is respect and trust toward 'Jugong clan rules'〔周公宗法〕and Neo-Confucian school's perception of clan rules. 'Jugong clan rules', which refers to the principle of the clan organization that denied the Eun Kingdom's principle of younger brother succeeding older brother and absolutized the principle of eldest son succeeding the father, was established to secure the principle of eldest son succeeding his father. Neo-Confucian school was largely faithful to such rules. Second part is perception of reasonable ritual based on attitudes to think of something from another person's perspective. This refers to his attitude to think about the object of the ritual system through his interpretation of the ritual system based on attitudes to take another person's perspective. Third part is perception of rituals based on governing through courtesy. He was deeply concerned about theories on ritual not based on clan rules and sought governing through courtesy based on clan rules. Fourth part is perception of rituals that king and Confucian scholars should practice rituals in the same way. This means the operation of clan rules of king and Confucian scholars should be the same. This is considered as a method based on governing through courtesy presented by rustic literati of the 17th century. It showed rustic literati's desire for society where clan rules were faithfully observed and exhibited the advanced approach of rustic literati who lived ahead of their times.