I cannot. But we have no other way to determine what is most just other than our reasoning.
Except...perhaps revelation from God I believe the proof of which is beyond reasoning and realize is also beyond that which we can determine except perhaps in hindsight.
So, for those humans who do not except revelation as an actionable truth we are left not with what is most just or even more just but with our perception of what may be just given our limited powers of reason.
Can we agree though that there must be, despite our inability to definitively discern it, a most just path to take?
Justice is determined by people
No, justice is discerned by people and as shown, most imperfectly. People don't create Justice through their action. Their action is at best determined to be just or unjust by imperfectly applying reason to the situation. Now people may define what is meant by just or unjust but cannot by example since the term is an abstraction of what is moral and such abstractions can be defined but not definitively determined to fit a particular case as much as we would like to think or hope so. Innocent people have been "justly" sent to prison only to later change the terms used to unjustly not because people decided to redefine their actions but because they didn't define them properly to begin with .
even believers based on their interpretation of what god wants.
How we define Justice has nothing to do with what we say is just. What believers determine God wants does not define what is Just but is interpreted to BE just as we have defined it.
No one, even believers, agree on what justice is in all situations.
I think even believers can generally agree on what justice means. Whether or not that definition applies to a particular instance is another matter of which believers and non-believers are imperfectly subject to. I wonder where people got the notion that since it can be shown that there are differing believers having differing notions of what is just that that proves that all believers must be wrong? Because believers can be wrong does not prove all believers to be wrong. The same as applies to non-believers.
You can have goodness without badness.
No, we can only have an awareness of a present good only in comparison to an awareness of a potential bad and once we are aware we can actualize either as autonomous persons.
It is good to help a person in need only because the potential to not help them is bad. If the potential to not help them doesn't exist then we couldn't be aware of helping them being good. It would simply be an insincere action.
You can have perfection without imperfection.
Again, I don't think so. An awareness of perfection requires an awareness of imperfection. Once you are aware of either or in order to make either of those a thing we are capable of being able to be aware of they must be capable of actualization.
Once perfection is achieved whatever that means, it would have to include an awareness of imperfection and the loss of its potential actualization. Anything less would not be perfect. Assuming perfection included an awareness of it.
Say humans were created perfect, what ever that means, this would imply imperfection in humans never existed. If it was Gods intention to be glorified by making them aware of their own perfection that perfection would have to include being autonomous or else they wouldn't be perfected as aware creatures. And since being aware of what perfection is, these creatures would have to be aware of what imperfection is, and since these creatures in their perfected state would have to be autonomous that implies ability to actualize what they are aware of since if they couldn't they wouldn't be autonomous and perfect. Hence the necessity for the potential of imperfection and if that necessity isn't met then no perfection is possible in the aware creature. So any creature given awareness must of necessity be created less than perfect in an unperfected universe in order to achieve an awareness of perfection.
In heaven this will be the case right?
I'm not sure what the case will be in "heaven" but in order for aware creatures to be perfected imperfection must be realized as well.
Helping someone get up after they have fallen is still good no matter if there is evil in the world or not.
Why is helping someone get up considered good? If nothing is considered evil in the world then not helping them get up would be just as good. If no evil existed then it couldn't be considered bad for them to have fallen in the first place. Or they couldn't have fallen. If their situation is not bad then why is changing their situation considered to be good?