There is one.
Let's call it nature. Reality is a structure that is 'out there'. It is independent of anyone's mind.
We all have some, but how much we can trust any particular bit of it varies over a wide range.
Knowledge is a model of some bit of reality. We cannot know reality directly; we can only build up concepts that approximate (abstract) pieces of the structure through the working of our nervous system and body as our body interacts with other real things. Knowledge is therefore always tentative and uncertain to some degree. We can speak of the reliability of some piece of knowledge, but not of its truth, although that reliability may become high enough that we can use 'truth' as a convenient label. Our complex nervous system lets us build up concepts about concepts and identify principles in the structure of nature. These principles extend our knowledge very efficiently.
The benefit of knowledge is prediction of the events of nature and the outcome of our actions. Ultimately, knowledge and belief are the same thing.
People are well-defined units.
We are are all similar (of course, that's what makes us a 'we'), but each of us is unique and each of us, ultimately, acts on our own and to our own purposes. We know that we exist. We have a sense and concept of our selves. Because we are well delineated as physical objects by our skins, it is easy to assign the direct results of our actions to us individually.
Our actions are driven most basically by needs of the body and secondarily by goals that we develop by interaction of our needs and our model of nature. Our selves are layered. The brain, the substrate for the self, has been built up through evolution by adding structures that give us new capabilities on top of the stuff that existed before. The self is a composite of things that range from parts that are common to all brained animals to parts that are unique to us. The newest layer in the brain, the frontal lobe features offer mainly the ability to place future benefit over immediate benefit. A lot of the specialness that we attribute to our selves may just be naive modeling.
Any world in which there are at least two individuals is a society.
Individuals have different and changing requirements and desires with respect to nature and with respect to each other.
Individual behavior is changed dramatically in a society. Similarities of need induce cooperation and competition and differences of need induce conflict of interest, all of which interact to create a rich dynamic of social behavior.
We all have it and it cannot escape us. Without society, there is none. We need someone else to be responsible to.
Responsibility is the obligation an individual feels to restrict his behavior so as to increase the advantages of cooperation and competition and to decrease the bad effects of conflict of interest in a society. An individual acting in a world without other people has no responsibility.
The problem with knowing what is responsible behavior and what is not is much more difficult (unreliable) for an individual than the problem of knowing simple nature because responsibility deals with much higher levels of abstraction. The problem of defining responsibility for a society is multiplied by the number of individuals in that society. Models of social expectations for responsible behavior build up over long periods of time in the form of mores, values, customs and laws. Socialization is the process in which an individual models what her society expects of her.
It's there and it isn't.
Individuals experience very positive feelings in reaction to certain senses and perceptions of nature, and, maybe in weaker degree, to perceptions of certain products of mind. The strength and importance of those feelings probably varies among individuals and within individuals over time as well. While the feelings seem to be universal, the things that stimulate the feelings seem to be much less so. The reaction to beauty also seems to be influenced by society to a large or small extent.
Nearly everyone feels it naturally, like nearly everyone sees the color blue. Language tries to imprison it. Definers try to distort it.
In the end, we might have to accept that human love has as many definitions as there are people multiplied by the years or even days in their lives. We seem to have great need to express love and discuss it, yet are foiled in every attempt to say exactly, or even most approximately, what it is. Love is much like beauty. Love may be beauty mixed with mind and body.
It comes with more certainty and less purpose than anything nature does to us.