For a review of how this question is debated in Cognitive Science, search for Searle's Chinese Room Thought Experiment.
In the Chinese Room Thought Experiment, Searle argues there is something fundamentally meaningful (semantic-holding-preserving) about the internal state of a living being. Additionally, this meaning cannot be approximated by a computer. Consequently, since this meaning cannot be approximated, the human being cannot be reduced to a function. Any attempt at this will fail to approximate this "meaning" and be incomplete in some respect.
To justify this stance, he makes the analogy between a person in a room that receives commands through a door in Chinese. Although the person does not understand Chinese, thanks to reference books, he can give the appropriate response on a piece of paper and slip it back under the door. In the analogy, the reference books are the database of rules, the person is the program and the door is the input and output. Obviously the program (person) does not understand Chinese or anything about the meaning of the input and output. How can any computer program (or as you say in your question "function") be said to approximate the human mind if it doesn't understand anything?
There are many replies to this thought experiment. The most potent reply I've come across to this is the System's Reply, which claims that although the person does not understand Chinese, the system as a whole is what understands Chinese.
Searle replies to this argument by letting the man internalize the books and let him wander outside into China. Searle claims that the man will still not understand Chinese in any "meaningful" manner.
I don't find this reply convincing since, when you look at what "meaningful" means to Searle and where it comes from biologically, Searle justifies that there is something in the synapses that can't be captured by a computer. This is an ineffective argument since it can't be proven or disproven by measurement or experiment. Additionally, it is easily defeated by Pylyshyn's reductionist argument. Pylyshyn argues that according to Searle, if you individually replaced every synapse with an identically functioning silicon component, eventually you would cease to have meaningful thought, which seems absurd.
Personally, I would argue that Searle's counter-argument to the System's Reply is a limited metaphor. Give the person a chance to modify the books in their head as well as interact with Chinese in the wild and undoubtedly they will understand Chinese! That's literally how people learn language!
Although I have clearly chosen a side in this debate, it's far from settled, as most things remain in philosophy, but hopefully this gives you a good starting point.