After several more papers in the 1970s, the theory was given a book-length treatment in 1980, "Exploring Meinong's Jungle and Beyond". The view—also defended in recent years by Priest—utilizes a modal theory including "impossible worlds" to deal with supposed objects, like the "round square." Sylvan's formulation is logically consistent, and avoids certain paradoxes associated with Meinong's original ontology; although, like many Meinongian views, it faces criticism due to its presumed ontological implausibility. Outside of logic and metaphysics, Sylvan was a proponent of so-called deep environmental ethics in the study of environmental ethics (following shallow/deep distinction formulated by Arne Næss). In his important 1973 paper "Is There a Need for a New, an Environmental, Ethic?", he defended a then-unorthodox account of the intrinsic value of the non-human, natural world. For this he was sometimes considered a defender of deep ecology, but he was in fact very critical of much of the domain. Beginning in the 1970s, Sylvan published several other notable articles and books on environmental ethics and issues, and he co-authored the 1994 book "The Greening of Ethics" with David Bennett. From his work in environmental ethics, Sylvan took an interest in anarchism, contributing an often-cited entry on the subject to "A Companion to Contemporary Political Philosophy".