The so-called information processing viewpoint is to compare the human brain with a computer, and treat the human brain as an information processing system similar to a computer. However, this analogy is only functional, that is, an analogy at the behavior level, regardless of its substance. The difference between biological cells and electronic components. In other words, the analogy involves only software and not hardware. "Hardware" in this passage refers to:. ['Functional properties.', 'Behavior level.', 'Material composition.', 'Biological cells and electronic components.']