In science, concepts such as organism, evolution and life, are used almost every day. Every scientist knows the general meaning of such concepts. At the same time, nature is complex, and for this reason, it is difficult to draw stringent lines around classes of things. Scientists therefore accept the use of so called 'working definitions' for many concepts. It is frequently advocated that working on definitions has little use for practical research. This book explores a different viewpoint, in which definitions are compared with tools. If your toolbox contains too few tools, tools that are worn down, or tools that don't fit, it becomes difficult to carry out even the most easy maintenance or repair job. Experts know: suitable tools make the work easier. The aim of this book is to examine much-used concepts in science as if these are tools in a scientific toolbox. Do the current definitions represent quality tools? To explore this question, this book uses a recently developed hierarchy theory, the operator theory, as a reference. This theory is explained in the first chapter. Whenever the analyses suggest to do so, the ScienceBites offer directions for improvement of current definitions. 'These delicious bites of science will inspire readers to devour much more scientific knowledge, and to reflect on the importance for humanity of progress in the sciences.' Prof. Dr Herman Philipse Utrecht University 'In this "golden age of biology" SCIENCEBITES provides a much needed critical reflection on its core terms' Prof. Dr Bart Gremmen Wageningen University & Research 'This is one of the most fascinating books I have read recently. Do not be deceived by the brevity of the chapters. Each gave me food for thought during many days.' Dr Peter Roessingh University of Amsterdam.