You need to know about this vase. It is known as The Rubin Vase or simply Rubin's Vase. It is also called the figure-ground vase.
On this site we will simply call it The Vase. Or The Curious Vase. It is important enough to carry such a generic name.
The reason for its name is that the vase was discovered by the Danish psychologist Edgar Rubin in 1915. Not deep in the ground but right in front of his eyes.
It has to do with the ambiguity of certain visual images. In this context, it has to do with the ambiguity of certain street photographs. And it has to do with figure and ground.
The point it that an image may change content depending on what you see as figure and what you see as ground. Look at the left illustration above and you will probably see a yellow vase on a white background. Look at the right illustration and you will likely recognize the profile of two men turned towards each other.
On the other hand you may even see quite the opposite or nothing meaningful at all. It shows you how complex visual communication can be. Even when it seemingly is very simple. How difficult can the picture of a vase be?
Here are Edgar Rubin's words: "One can then state as a fundamental principle: When two fields have a common border, and one is seen as figure and the other as ground, the immediate perceptual experience is characterized by a shaping effect which emerges from the common border of the fields and which operates only on one field or operates more strongly on one than on the other."
The implication of this is that one of the two possible impression will always come out stronger than the other. I would say that it is hardly possible to have the two impressions precent at the same time. It can be done, but it is definitely a strain both on your eyes and to your brain.
Try for yourself. Concentrate on the left illustration. What do you see? The vase or the profiles? Are you able to shift between the two impressions at your own will, or are you completely stuck with one and unable so see the other? Are you able to see both the vase and the profiles at the same time? If so, you are very good.
Try the same with the right illustration. Might be easier.
The idea is very simple: You have to be careful with ambiguous images if you want the viewer to get your message. That is, if this is your intention at all as a photographer. You may not be in the message business. Most of us aren't. Even so, your street images always communicate something. The question is if you want to be in control or not.
Implications for street photography
Why is the curious vase important for street photographers? I am sure that you have already answered this. Maybe I just did.
If you aim at creating clear and district messages you should avoid sending ambiguous signals. The vase is even easy to read compared to the multitude of information you can have in a single street photograph where many things are outside your control. That is some of the beauty of street photography and why so many enjoy it. You don't know what you get till you have it in the box and can bring it home with you.
Nevertheless, try to keep your photographs simple. Go for one message only. Make sure that the part of reality that you capture is framed in such a way that people have no or little problem understanding it. It is very easy to confuse the human mind.
Have a look at pictures made by Henri Cartier-Bresson, Robert Frank, Walker Evans or any other master. See how sharp these guys are in their rendering of messages. Even if they are not always on a message mission.
They are very, very sharp. Trust me. Or see for yourself.
Try it out
You may want to try this: The next time you wander the streets hunting photographic game try to take some pictures that are ambiguous. That done, try to shoot some that are definitely not ambiguous. Go for the vase or the profile and then for both. Know what you are doing.
Ask people what they think about your pictures shot at that special session. Load them to a social site and see if people like them or not. Which do they like better?
Now you know about The Curious Vase. And the two profiles. Not all about it, but some. Enough.
Good luck checking it out the curious vase with your camera. It does not have to be a (.....), but it certainly helps.