What’s an instruction anyway?

I really like this Wired article and the underlying study in Science because the research brings computing and storage into context, both historically and with respect to types of devices.  

From the abstract, "In 2007, humankind was able to store 2.9 × 1020 optimally compressed bytes, communicate almost 2 × 1021 bytes, and carry out 6.4 × 1018 instructions per second on general-purpose computers."  In perspective, the total number of instructions is only a single magnitude of order larger than the number of nerve impulses in the brain per second.  Furthermore, the total amount of storage is roughly the number of bits stored in the DNA of a human adult.  

However, I think it’s important to note that not all instructions are created equal.  This point is easily made by comparing RISC and CISC CPU designs.  So before we really stretch the comparison between nerve impulses and instructions, I think we need to actually define an instruction (and learn a hell of a lot about the brain).