The staining of neurotransmitter substances is possible in postmortem human material as well as in animals.
The on-line and/or in situ determination of some substances is possible, but expensive, as sample collection and pre-treatment are often necessary with strict rules of sterility.
One path is skepticism, the view that without certainty, no knowledge of substances is possible at all.
The principle of coexistence then argues that harmonious causal interaction between otherwise isolated, independently existing substances is possible only by means of God's coordination (just as Leibniz thought was required for harmonious relations between the states of such substances).
The conclusion that no intuitive or demonstrative knowledge of substances is possible for us, their real essences and necessary connections being out of reach, places Locke at a crossroads.
Random variables associated with arcs can be particularly useful to express general knowledge about molecular events when it is known that an interaction between two substances is possible, but no precise specification of the condition is given.
Activation of organic substances is possible, as well as formation of peptide bonds between amino acids, and phosphodiester (PDE) bonds between nucleotides.
One concerns acting freely for a reason and the reason-explanation of free actions; the second is the problem of luck; the last concerns the intelligibility of the notion of agent causation and whether causation by an agent (where this is understood to be causation by a substance) is possible.
But, because all animals exhibit some general ability to recognize self and to repel foreign substances, it is possible to study the immune capacity of living animals and, based on the relative positions of these animals in the evolutionary tree, to extrapolate a reasonable evolutionary history of the immune system.
For therapeutic injections of large joints in patients with allergy to contrast substances, it is possible to inject a small volume of air (3-5 ml) as a substitute.
