It was easy doing some other things aside from baby sitting him. However, when he reached the age of about one nine months or so, when he clearly recognizes me and reacts positively upon seeing me, I have observed that it was harder leaving him alone on a crib since he wails whenever he does not see me somewhere near him. I have found the situation difficult since it prevented me from doing other important things that I was supposed to do. He was no longer contented on the things I give him to keep him busy whenever I was not around. He would cry out loud when he did not see me around him. I have observed this incident for a number of times until my nephew was around more than a year old.
The same situation happens with some of our neighbors' babies at the same stage as my nephew had exhibited that particular behavior. The same behaviors happen to the babies having different backgrounds. Therefore, no one can drive into a conclusion that these behaviors of babies are influenced by their backgrounds - an inherited behavior from parents or a controlled behavior by someone who takes care of them.
An elaboration of these behaviors in the babies' developmental stage was theorized by Piaget as the concept of the Object of Permanence. According to Piaget, the incident of object permanence develops during the sensory-motor stage of cognitive development of infants. Object of permanence refers to the ability of the brain to retain and utilize visual images. It is used to describe the awareness that objects continue to exist even when they are no longer visible. This situation is distinct from a baby's recognition memory. Piaget's experiment with the infants made him conclude that awareness is typically achieved on their eighth to ninth month of age. When these infants become aware of the objects around him, separation anxiety happens simultaneously. (http://en.wikipedia.org/wiki/Object _permanence and http://www.drhull.com/EncyMaster/O/object_permanence.html, May 6, 2006