Sacrifice requires presence: you should sacrifice to the spirits as if they are there. Confucius said: “If I am not fully present at the sacrifice, it’s as if I didn’t attend the sacrifice at all.”
Often when I go to conferences and see other attendees openly hammering away on their laptops or covertly checking their email and social media feeds on their smart phone or tablet during a presentation, I do find myself wondering why they are bothering to attend. How can you listen properly to what the speaker has to say if your mind is elsewhere?
Confucius is making exactly the same point about the need to be “fully present” at the sacrificial ceremony. If you’re there in body but not mind and spirit then it’s better not to be there at all.