This collection of essays, along with Rosen's other book _Life Itself_, are mandatory reading for any scientist or any astute layperson interested in biology, physics or philosophy of science.Rosen was a very insightful and technically capable theoretical biologist. His work - first as a student of physicist and theoretical biologist Nicholas Rashevsky, and later as professor emeritus at Dalhousie - is unquestionably of the level of importance of Einstein's Special/General Theory of Relativity, or Godel's Incompleteness Theorems. This is a grand claim to make, but once you read Rosen's work, you will see for yourself.These are not the easiest books to read, despite Rosen's excellent writing skills. The difficulty is two-fold. First and foremost, the new concepts and paradigms presented are of such breadth and profundity that it can take several readings to begin to fully grasp them adequately. Secondly, Rosen is mathematically (and otherwise) quite astute. The reader will encounter to some degree: category theory, topology, catastrophe theory (Rosen dedicates a chapter on genericity in _Essays_ to Rene Thom), differential equations, dynamical systems, Godel, Church-Turing, as well as philosophical topics of epistemology, ontology, and foundations of biology, mathematics and physics.This should not, however, deter even the non-professional. Particularly in _Life Itself_, Rosen progresses carefully and patiently, even including a short intro to Category Theory. One can gloss over some of the math and still garner most of the insights from the text alone. _Essays_ utilizes a wider range of math skills, since that book covers a broader range of topics, but it is still quite accessible to the careful and astute reader.In _Life Itself_, Rosen was investigating the question posed by Erwin Shrodinger originally in his 1943 lecture "What is Life?". Rosen's search led him to peel back in careful detail the foundations of Newtonian mechanics and reveal the underlying tacit assumptions of a state/phase-based physics and the repercussions for science in general, and biology in particular.By setting aside state/phase-based physics, Rosen then proceeded to layout the groundwork for an atemporal relational biology based on functional organization and to methodically investigate the theoretical limits of mechanistic systems, including along the way: simulation, Turing machines, and the epistemology and ontology of such systems. The distinction eventually becomes clear that any such algorithmic mechanisms cannot embody the kinds of impredicative complexity that are characteristic of an organism. Because the syntax of Newtonian physics can express no such closed loops of entailment, "life" cannot even be described in that model of physics, much less modeled in any complete way. Thus it is that biological organisms are not a mere subset of current physics, but are representative of complexities that require physics to be enlarged.In _Essays on Life Itself_, Rosen uses his considerable abilities across a broad spectrum of topics to continue the ideas from _Life Itself_. It is difficult to describe how topics as diverse as the assumptions of Pythagoras, the Turing test, universal unfoldings, morphogenesis, mind-brain problem, and more can be in the same book. Mostly, they all in one way or another accomplish one task: to look beyond the limits of how a problem is currently being viewed, and to see it from a larger perspective. Often, these perspectives take Rosen into terrain others would avoid, since they sometimes lead into the non-algorithmic / noncomputable, or the breakdown of the presumed subject-object division, or other kinds of "messy" scenarios.Often they lead into "complex systems", where Rosen uses the word "complex" to define a certain class of systems - those systems have symptoms of being: impredicative, non-algorithmic, context-dependent, semantic, nonformalizable. This classification is not a desire for obfuscation or ineffability, but is as rigorous as the nonformalizability of Number Theory or the unsolvability in closed form of the n-body problem. It is a complexity akin to the size of a transfinite number: it is not simply a matter of merely being hugely complicated, it is rather an entirely different order of system structure.However, guided by Rosen, one does not feel uneasy following his path. Rather one feels enriched both in knowledge and in paradigm. Distinguishing the broader generic case from the degenerate or special is a characteristic theme in Rosen. The unfamiliar terrain he argues to is thus not some void, but a grander scale that subsumes the orthodox view.In that grander view, it may become more clear that some problems are based on incorrect assumptions, while some are more difficult or complex than in the more limited original view. However, it is apparent that Rosen is uninterested in making problems appear simpler by ignoring those difficulties - he is interested in where the science leads. It is an immensely richer, complex view of the physical world that one comes away with. As such, it presents some difficult challanges, but it also opens up vast opportunities - opportunities not visible in the neat and tidy fantasy model of science that generally prevails where it is assumed that with enough effort everything can be reduced or calculated.Rosen writes deliberately and with precision, and is both a critical and a profound thinker. I hope that he one day receives the recognition and admiration he rightfully deserves.