Well, for the last several decades, biologists, the true experts on the question of life (in the physical sense) have used certain criteria to separate living from inanimate objects, as well as organism from tissue, These criteria are compiled in the marvelous introductory biology text by Helena Curtis. Biology (Worth Publishers Inc., 2nd ed., 1976).
1.”Living organisms are complex and highly organized.” As you can see, for example, in figure 1, in the highly complex arrangement of the cytoskeleton (‘cell skeleton’). This is the ‘scaffolding’ along which the processes inside the cell are organized.
2.”Living thing take energy from their environment and change it from one form to another.” We eat and drink, and turn the intake into heat and energy for exercise.
3. “Living things are homeostatic”. Homeostatic means to stay the same. For example, our temperature, blood pH and need for water generally (in health) remain at precise levels.
4.”Living things respond to stimuli”. Pinch someone hard: they’ll tell you about it. Shout loud, they’ll hear. Put a lot of pepper on their food, you know something will happen.
5. “Living things reproduce themselves”. This is where an unfertilized egg cell falls short of being a living organism. It cannot reproduce itself in any manner.
6. “Living things grow and develop”. Unfertilized eggs and blobs cannot do this. A fertilized egg cell can qualify in this category, of course.
7. “Living things are adapted”. This requires a bit of conceptual biology, but amounts to: living things are shaped to work well in their environment. Blobs don’t make it.
8. Finally, “The information by which living things organize their purposeful structures and functions, maintain homeostasis, convert energy, respond to stimuli, reproduce and develop, is all contained within the individual organism itself.”
The last point leads to many concepts in biology, but I’ll mention these few: unfertilized eggs lack exactly half of the above information; from conception through to death, the fertilized egg as well as the grown adult share the above information; tissue like the liver, for example, are living, but are not individual organisms because they lack all the above information.
It is important to realize that a living organism expresses all these traits, and if any one is missing – it is not considered alive. To deny that the fertilized human egg is anything but human is to deny reality utterly.