My intuition on _what is a **probe**_ derives from my own background with programming and graphs. To me it sounds like an index into an array, or a pointer to a single object in some collection of objects. At different times, the index or pointer or arrow or morphism may target something different. I have to draw some diagrams to demonstrate what I'm thinking, which I believe is pretty much what @EricRogstad suggests.

```
// Probe at some time t
digraph G { graph[style=dotted];
subgraph clusterA {p}
subgraph clusterB {1 2 3 4}
p->2
}
```

![Figure G](https://gist.githubusercontent.com/kenwebb/b8a7893bc4723388cd6e355069c9fd55/raw/fa53e8fc74845d887fd5cde8a9cda27642bf4893/probeG.svg?sanitize=true "Figure G")

```
// Probe at some other time t'
digraph H { graph[style=dotted];
subgraph clusterA {p}
subgraph clusterB {1 2 3 4}
p->1
}
```

![Figure H](https://gist.githubusercontent.com/kenwebb/b8a7893bc4723388cd6e355069c9fd55/raw/fa53e8fc74845d887fd5cde8a9cda27642bf4893/probeH.svg?sanitize=true "Figure H")

```
// Selecting 2 elements from a set
digraph I { graph[style=dotted];
subgraph clusterA {p q}
subgraph clusterB {1 2 3 4}
p->1; q->4;
}
```

![Figure I](https://gist.githubusercontent.com/kenwebb/b8a7893bc4723388cd6e355069c9fd55/raw/fa53e8fc74845d887fd5cde8a9cda27642bf4893/probeI.svg?sanitize=true "Figure I")

I used [WebGraphviz](http://www.webgraphviz.com/) to generate the images from the Graphviz notation.